updated core from 8.4 to 8.5 : bug with login_destination

core/modules/layout_builder/config/schema/layout_builder.schema.yml

@@ -0,0 +1,46 @@
+core.entity_view_display.*.*.*.third_party.layout_builder:
+  type: mapping
+  label: 'Per-view-mode Layout Builder settings'
+  mapping:
+    allow_custom:
+      type: boolean
+      label: 'Allow a customized layout'
+    sections:
+      type: sequence
+      sequence:
+        type: layout_builder.section
+
+layout_builder.section:
+  type: mapping
+  label: 'Layout section'
+  mapping:
+    layout_id:
+      type: string
+      label: 'Layout ID'
+    layout_settings:
+      type: layout_plugin.settings.[%parent.layout_id]
+      label: 'Layout settings'
+    components:
+      type: sequence
+      label: 'Components'
+      sequence:
+        type: layout_builder.component
+
+layout_builder.component:
+  type: mapping
+  label: 'Component'
+  mapping:
+    configuration:
+      type: block.settings.[id]
+    region:
+      type: string
+      label: 'Region'
+    uuid:
+      type: uuid
+      label: 'UUID'
+    weight:
+      type: integer
+      label: 'Weight'
+    additional:
+      type: ignore
+      label: 'Additional data'

core/modules/layout_builder/css/layout-builder.css

@@ -0,0 +1,51 @@
+.add-section {
+  width: 100%;
+  outline: 2px dashed #979797;
+  padding: 1.5em 0;
+  text-align: center;
+  margin-bottom: 1.5em;
+  transition: visually-hidden 2s ease-out, height 2s ease-in;
+}
+
+.layout-section {
+  margin-bottom: 1.5em;
+}
+
+.layout-section .layout-builder--layout__region {
+  outline: 2px dashed #2f91da;
+  padding: 1.5em 0;
+}
+
+.layout-section .layout-builder--layout__region .add-block {
+  text-align: center;
+}
+
+.layout-section .remove-section {
+  position: relative;
+  background: url(../../../misc/icons/bebebe/ex.svg) #fff center center / 16px 16px no-repeat;
+  border: 1px solid #ccc;
+  box-sizing: border-box;
+  font-size: 1rem;
+  padding: 0;
+  height: 26px;
+  width: 26px;
+  white-space: nowrap;
+  text-indent: -9999px;
+  display: inline-block;
+  border-radius: 26px;
+  margin-left: -10px;
+}
+
+.layout-section .remove-section:hover {
+  background-image: url(../../../misc/icons/787878/ex.svg);
+}
+
+#drupal-off-canvas .layout-selection li {
+  display: block;
+  padding-bottom: 1em;
+}
+
+#drupal-off-canvas .layout-selection li a {
+  display: block;
+  padding-top: 0.55em;
+}

core/modules/layout_builder/js/layout-builder.es6.js

@@ -0,0 +1,41 @@
+(($, { ajax, behaviors }) => {
+  behaviors.layoutBuilder = {
+    attach(context) {
+      $(context).find('.layout-builder--layout__region').sortable({
+        items: '> .draggable',
+        connectWith: '.layout-builder--layout__region',
+
+        /**
+         * Updates the layout with the new position of the block.
+         *
+         * @param {jQuery.Event} event
+         *   The jQuery Event object.
+         * @param {Object} ui
+         *   An object containing information about the item being sorted.
+         */
+        update(event, ui) {
+          // Check if the region from the event and region for the item match.
+          const itemRegion = ui.item.closest('.layout-builder--layout__region');
+          if (event.target === itemRegion[0]) {
+            // Find the destination delta.
+            const deltaTo = ui.item.closest('[data-layout-delta]').data('layout-delta');
+            // If the block didn't leave the original delta use the destination.
+            const deltaFrom = ui.sender ? ui.sender.closest('[data-layout-delta]').data('layout-delta') : deltaTo;
+            ajax({
+              url: [
+                ui.item.closest('[data-layout-update-url]').data('layout-update-url'),
+                deltaFrom,
+                deltaTo,
+                itemRegion.data('region'),
+                ui.item.data('layout-block-uuid'),
+                ui.item.prev('[data-layout-block-uuid]').data('layout-block-uuid'),
+              ]
+              .filter(element => element !== undefined)
+              .join('/'),
+            }).execute();
+          }
+        },
+      });
+    },
+  };
+})(jQuery, Drupal);

core/modules/layout_builder/js/layout-builder.js

@@ -0,0 +1,34 @@
+/**
+* DO NOT EDIT THIS FILE.
+* See the following change record for more information,
+* https://www.drupal.org/node/2815083
+* @preserve
+**/
+
+(function ($, _ref) {
+  var ajax = _ref.ajax,
+      behaviors = _ref.behaviors;
+
+  behaviors.layoutBuilder = {
+    attach: function attach(context) {
+      $(context).find('.layout-builder--layout__region').sortable({
+        items: '> .draggable',
+        connectWith: '.layout-builder--layout__region',
+
+        update: function update(event, ui) {
+          var itemRegion = ui.item.closest('.layout-builder--layout__region');
+          if (event.target === itemRegion[0]) {
+            var deltaTo = ui.item.closest('[data-layout-delta]').data('layout-delta');
+
+            var deltaFrom = ui.sender ? ui.sender.closest('[data-layout-delta]').data('layout-delta') : deltaTo;
+            ajax({
+              url: [ui.item.closest('[data-layout-update-url]').data('layout-update-url'), deltaFrom, deltaTo, itemRegion.data('region'), ui.item.data('layout-block-uuid'), ui.item.prev('[data-layout-block-uuid]').data('layout-block-uuid')].filter(function (element) {
+                return element !== undefined;
+              }).join('/')
+            }).execute();
+          }
+        }
+      });
+    }
+  };
+})(jQuery, Drupal);

core/modules/layout_builder/layout_builder.info.yml

@@ -0,0 +1,17 @@
+name: 'Layout Builder'
+type: module
+description: 'Provides layout building utility.'
+package: Core (Experimental)
+# version: VERSION
+# core: 8.x
+dependencies:
+  - layout_discovery
+  - contextual
+  # @todo Discuss removing in https://www.drupal.org/project/drupal/issues/2935999.
+  - field_ui
+
+# Information added by Drupal.org packaging script on 2018-03-07
+version: '8.5.0'
+core: '8.x'
+project: 'drupal'
+datestamp: 1520457825

core/modules/layout_builder/layout_builder.install

@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * @file
+ * Contains install and update functions for Layout Builder.
+ */
+
+use Drupal\Core\Cache\Cache;
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay;
+use Drupal\layout_builder\Section;
+
+/**
+ * Implements hook_install().
+ */
+function layout_builder_install() {
+  $displays = LayoutBuilderEntityViewDisplay::loadMultiple();
+  /** @var \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface[] $displays */
+  foreach ($displays as $display) {
+    // Create the first section from any existing Field Layout settings.
+    $field_layout = $display->getThirdPartySettings('field_layout');
+    if (isset($field_layout['id'])) {
+      $field_layout += ['settings' => []];
+      $display->appendSection(new Section($field_layout['id'], $field_layout['settings']));
+    }
+
+    // Sort the components by weight.
+    $components = $display->get('content');
+    uasort($components, 'Drupal\Component\Utility\SortArray::sortByWeightElement');
+    foreach ($components as $name => $component) {
+      $display->setComponent($name, $component);
+    }
+    $display->save();
+  }
+
+  // Clear the rendered cache to ensure the new layout builder flow is used.
+  // While in many cases the above change will not affect the rendered output,
+  // the cacheability metadata will have changed and should be processed to
+  // prepare for future changes.
+  Cache::invalidateTags(['rendered']);
+}

core/modules/layout_builder/layout_builder.libraries.yml

@@ -0,0 +1,10 @@
+drupal.layout_builder:
+  version: VERSION
+  css:
+    theme:
+      css/layout-builder.css: {}
+  js:
+    js/layout-builder.js: {}
+  dependencies:
+    - core/jquery.ui.sortable
+    - core/drupal.dialog.off_canvas

core/modules/layout_builder/layout_builder.links.contextual.yml

@@ -0,0 +1,19 @@
+layout_builder_block_update:
+  title: 'Configure'
+  route_name: 'layout_builder.update_block'
+  group: 'layout_builder_block'
+  options:
+    attributes:
+      class: ['use-ajax']
+      data-dialog-type: dialog
+      data-dialog-renderer: off_canvas
+
+layout_builder_block_remove:
+  title: 'Remove block'
+  route_name: 'layout_builder.remove_block'
+  group: 'layout_builder_block'
+  options:
+    attributes:
+      class: ['use-ajax']
+      data-dialog-type: dialog
+      data-dialog-renderer: off_canvas

core/modules/layout_builder/layout_builder.links.task.yml

@@ -0,0 +1,2 @@
+layout_builder_ui:
+  deriver: '\Drupal\layout_builder\Plugin\Derivative\LayoutBuilderLocalTaskDeriver'

core/modules/layout_builder/layout_builder.module

@@ -0,0 +1,83 @@
+<?php
+
+/**
+ * @file
+ * Provides hook implementations for Layout Builder.
+ */
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Url;
+use Drupal\field\FieldConfigInterface;
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay;
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplayStorage;
+use Drupal\layout_builder\Form\LayoutBuilderEntityViewDisplayForm;
+
+/**
+ * Implements hook_help().
+ */
+function layout_builder_help($route_name, RouteMatchInterface $route_match) {
+  // Add help text to the Layout Builder UI.
+  if ($route_match->getRouteObject()->getOption('_layout_builder')) {
+    $output = '<p>' . t('This layout builder tool allows you to configure the layout of the main content area.') . '</p>';
+    if (\Drupal::currentUser()->hasPermission('administer blocks')) {
+      $output .= '<p>' . t('To manage other areas of the page, use the <a href="@block-ui">block administration page</a>.', ['@block-ui' => Url::fromRoute('block.admin_display')->toString()]) . '</p>';
+    }
+    else {
+      $output .= '<p>' . t('To manage other areas of the page, use the block administration page.') . '</p>';
+    }
+    return $output;
+  }
+
+  switch ($route_name) {
+    case 'help.page.layout_builder':
+      $output = '<h3>' . t('About') . '</h3>';
+      $output .= '<p>' . t('Layout Builder provides layout building utility.') . '</p>';
+      $output .= '<p>' . t('For more information, see the <a href=":layout-builder-documentation">online documentation for the Layout Builder module</a>.', [':layout-builder-documentation' => 'https://www.drupal.org/docs/8/core/modules/layout_builder']) . '</p>';
+      return $output;
+  }
+}
+
+/**
+ * Implements hook_entity_type_alter().
+ */
+function layout_builder_entity_type_alter(array &$entity_types) {
+  /** @var \Drupal\Core\Entity\EntityTypeInterface[] $entity_types */
+  $entity_types['entity_view_display']
+    ->setClass(LayoutBuilderEntityViewDisplay::class)
+    ->setStorageClass(LayoutBuilderEntityViewDisplayStorage::class)
+    ->setFormClass('edit', LayoutBuilderEntityViewDisplayForm::class);
+}
+
+/**
+ * Implements hook_form_FORM_ID_alter() for \Drupal\field_ui\Form\EntityFormDisplayEditForm.
+ */
+function layout_builder_form_entity_form_display_edit_form_alter(&$form, FormStateInterface $form_state) {
+  // Hides the Layout Builder field. It is rendered directly in
+  // \Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay::buildMultiple().
+  unset($form['fields']['layout_builder__layout']);
+  $key = array_search('layout_builder__layout', $form['#fields']);
+  if ($key !== FALSE) {
+    unset($form['#fields'][$key]);
+  }
+}
+
+/**
+ * Implements hook_field_config_insert().
+ */
+function layout_builder_field_config_insert(FieldConfigInterface $field_config) {
+  // Clear the sample entity for this entity type and bundle.
+  $sample_entity_generator = \Drupal::service('layout_builder.sample_entity_generator');
+  $sample_entity_generator->delete($field_config->getTargetEntityTypeId(), $field_config->getTargetBundle());
+  \Drupal::service('plugin.manager.block')->clearCachedDefinitions();
+}
+
+/**
+ * Implements hook_field_config_delete().
+ */
+function layout_builder_field_config_delete(FieldConfigInterface $field_config) {
+  // Clear the sample entity for this entity type and bundle.
+  $sample_entity_generator = \Drupal::service('layout_builder.sample_entity_generator');
+  $sample_entity_generator->delete($field_config->getTargetEntityTypeId(), $field_config->getTargetBundle());
+  \Drupal::service('plugin.manager.block')->clearCachedDefinitions();
+}

core/modules/layout_builder/layout_builder.permissions.yml

@@ -0,0 +1,5 @@
+# @todo Expand permissions to be more granular in
+#   https://www.drupal.org/node/2914486.
+configure any layout:
+  title: 'Configure any layout'
+  restrict access: true

core/modules/layout_builder/layout_builder.post_update.php

@@ -0,0 +1,26 @@
+<?php
+
+/**
+ * @file
+ * Post update functions for Layout Builder.
+ */
+
+/**
+ * Rebuild plugin dependencies for all entity view displays.
+ */
+function layout_builder_post_update_rebuild_plugin_dependencies(&$sandbox = NULL) {
+  $storage = \Drupal::entityTypeManager()->getStorage('entity_view_display');
+  if (!isset($sandbox['ids'])) {
+    $sandbox['ids'] = $storage->getQuery()->accessCheck(FALSE)->execute();
+    $sandbox['count'] = count($sandbox['ids']);
+  }
+
+  for ($i = 0; $i < 10 && count($sandbox['ids']); $i++) {
+    $id = array_shift($sandbox['ids']);
+    if ($display = $storage->load($id)) {
+      $display->save();
+    }
+  }
+
+  $sandbox['#finished'] = empty($sandbox['ids']) ? 1 : ($sandbox['count'] - count($sandbox['ids'])) / $sandbox['count'];
+}

core/modules/layout_builder/layout_builder.routing.yml

@@ -0,0 +1,117 @@
+layout_builder.choose_section:
+  path: '/layout_builder/choose/section/{section_storage_type}/{section_storage}/{delta}'
+  defaults:
+   _controller: '\Drupal\layout_builder\Controller\ChooseSectionController::build'
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE
+
+layout_builder.add_section:
+  path: '/layout_builder/add/section/{section_storage_type}/{section_storage}/{delta}/{plugin_id}'
+  defaults:
+    _controller: '\Drupal\layout_builder\Controller\AddSectionController::build'
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE
+
+layout_builder.configure_section:
+  path: '/layout_builder/configure/section/{section_storage_type}/{section_storage}/{delta}/{plugin_id}'
+  defaults:
+    _title: 'Configure section'
+    _form: '\Drupal\layout_builder\Form\ConfigureSectionForm'
+    # Adding a new section requires a plugin_id, while configuring an existing
+    # section does not.
+    plugin_id: null
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE
+
+layout_builder.remove_section:
+  path: '/layout_builder/remove/section/{section_storage_type}/{section_storage}/{delta}'
+  defaults:
+    _form: '\Drupal\layout_builder\Form\RemoveSectionForm'
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE
+
+layout_builder.choose_block:
+  path: '/layout_builder/choose/block/{section_storage_type}/{section_storage}/{delta}/{region}'
+  defaults:
+    _controller: '\Drupal\layout_builder\Controller\ChooseBlockController::build'
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE
+
+layout_builder.add_block:
+  path: '/layout_builder/add/block/{section_storage_type}/{section_storage}/{delta}/{region}/{plugin_id}'
+  defaults:
+    _form: '\Drupal\layout_builder\Form\AddBlockForm'
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE
+
+layout_builder.update_block:
+  path: '/layout_builder/update/block/{section_storage_type}/{section_storage}/{delta}/{region}/{uuid}'
+  defaults:
+    _form: '\Drupal\layout_builder\Form\UpdateBlockForm'
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE
+
+layout_builder.remove_block:
+  path: '/layout_builder/remove/block/{section_storage_type}/{section_storage}/{delta}/{region}/{uuid}'
+  defaults:
+    _form: '\Drupal\layout_builder\Form\RemoveBlockForm'
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE
+
+layout_builder.move_block:
+  path: '/layout_builder/move/block/{section_storage_type}/{section_storage}/{delta_from}/{delta_to}/{region_to}/{block_uuid}/{preceding_block_uuid}'
+  defaults:
+    _controller: '\Drupal\layout_builder\Controller\MoveBlockController::build'
+    delta_from: null
+    delta_to: null
+    region_from: null
+    region_to: null
+    block_uuid: null
+    preceding_block_uuid: null
+  requirements:
+    _permission: 'configure any layout'
+  options:
+    _admin_route: TRUE
+    parameters:
+      section_storage:
+        layout_builder_tempstore: TRUE

core/modules/layout_builder/layout_builder.services.yml

@@ -0,0 +1,38 @@
+services:
+  layout_builder.tempstore_repository:
+    class: Drupal\layout_builder\LayoutTempstoreRepository
+    arguments: ['@tempstore.shared']
+  access_check.entity.layout:
+    class: Drupal\layout_builder\Access\LayoutSectionAccessCheck
+    tags:
+      - { name: access_check, applies_to: _has_layout_section }
+  plugin.manager.layout_builder.section_storage:
+    class: Drupal\layout_builder\SectionStorage\SectionStorageManager
+    parent: default_plugin_manager
+  layout_builder.routes:
+    class: Drupal\layout_builder\Routing\LayoutBuilderRoutes
+    arguments: ['@plugin.manager.layout_builder.section_storage']
+    tags:
+     - { name: event_subscriber }
+  layout_builder.route_enhancer:
+    class: Drupal\layout_builder\Routing\LayoutBuilderRouteEnhancer
+    tags:
+      - { name: route_enhancer }
+  layout_builder.param_converter:
+    class: Drupal\layout_builder\Routing\LayoutTempstoreParamConverter
+    arguments: ['@layout_builder.tempstore_repository', '@plugin.manager.layout_builder.section_storage']
+    tags:
+      - { name: paramconverter, priority: 10 }
+  cache_context.layout_builder_is_active:
+    class: Drupal\layout_builder\Cache\LayoutBuilderIsActiveCacheContext
+    arguments: ['@current_route_match']
+    tags:
+      - { name: cache.context}
+  layout_builder.sample_entity_generator:
+    class: Drupal\layout_builder\Entity\LayoutBuilderSampleEntityGenerator
+    arguments: ['@tempstore.shared', '@entity_type.manager']
+  layout_builder.render_block_component_subscriber:
+    class: Drupal\layout_builder\EventSubscriber\BlockComponentRenderArray
+    arguments: ['@current_user']
+    tags:
+      - { name: event_subscriber }

core/modules/layout_builder/src/Access/LayoutSectionAccessCheck.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\layout_builder\Access;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Routing\Access\AccessInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides an access check for the Layout Builder UI.
+ *
+ * @internal
+ */
+class LayoutSectionAccessCheck implements AccessInterface {
+
+  /**
+   * Checks routing access to the layout.
+   *
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The current route match.
+   * @param \Drupal\Core\Session\AccountInterface $account
+   *   The currently logged in account.
+   *
+   * @return \Drupal\Core\Access\AccessResultInterface
+   *   The access result.
+   */
+  public function access(RouteMatchInterface $route_match, AccountInterface $account) {
+    $section_storage = $route_match->getParameter('section_storage');
+
+    if (empty($section_storage)) {
+      return AccessResult::forbidden()->addCacheContexts(['route']);
+    }
+
+    if (!$section_storage instanceof SectionStorageInterface) {
+      $access = AccessResult::forbidden();
+    }
+    else {
+      $access = AccessResult::allowedIfHasPermission($account, 'configure any layout');
+    }
+
+    return $access->addCacheableDependency($section_storage);
+  }
+
+}

core/modules/layout_builder/src/Annotation/SectionStorage.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace Drupal\layout_builder\Annotation;
+
+use Drupal\Component\Annotation\Plugin;
+use Drupal\layout_builder\SectionStorage\SectionStorageDefinition;
+
+/**
+ * Defines a Section Storage type annotation object.
+ *
+ * @see \Drupal\layout_builder\SectionStorage\SectionStorageManager
+ * @see plugin_api
+ *
+ * @Annotation
+ */
+class SectionStorage extends Plugin {
+
+  /**
+   * The plugin ID.
+   *
+   * @var string
+   */
+  public $id;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function get() {
+    return new SectionStorageDefinition($this->definition);
+  }
+
+}

core/modules/layout_builder/src/Cache/LayoutBuilderIsActiveCacheContext.php

@@ -0,0 +1,87 @@
+<?php
+
+namespace Drupal\layout_builder\Cache;
+
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Cache\Context\CalculatedCacheContextInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+
+/**
+ * Determines whether Layout Builder is active for a given entity type or not.
+ *
+ * Cache context ID: 'layout_builder_is_active:%entity_type_id', e.g.
+ * 'layout_builder_is_active:node' (to vary by whether the Node entity type has
+ * Layout Builder enabled).
+ */
+class LayoutBuilderIsActiveCacheContext implements CalculatedCacheContextInterface {
+
+  /**
+   * The current route match.
+   *
+   * @var \Drupal\Core\Routing\RouteMatchInterface
+   */
+  protected $routeMatch;
+
+  /**
+   * LayoutBuilderCacheContext constructor.
+   *
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The current route match.
+   */
+  public function __construct(RouteMatchInterface $route_match) {
+    $this->routeMatch = $route_match;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getLabel() {
+    return t('Layout Builder');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContext($entity_type_id = NULL) {
+    if (!$entity_type_id) {
+      throw new \LogicException('Missing entity type ID');
+    }
+
+    $display = $this->getDisplay($entity_type_id);
+    return ($display && $display->isOverridable()) ? '1' : '0';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheableMetadata($entity_type_id = NULL) {
+    if (!$entity_type_id) {
+      throw new \LogicException('Missing entity type ID');
+    }
+
+    $cacheable_metadata = new CacheableMetadata();
+    if ($display = $this->getDisplay($entity_type_id)) {
+      $cacheable_metadata->addCacheableDependency($display);
+    }
+    return $cacheable_metadata;
+  }
+
+  /**
+   * Returns the entity view display for a given entity type and view mode.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   *
+   * @return \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface|null
+   *   The entity view display, if it exists.
+   */
+  protected function getDisplay($entity_type_id) {
+    if ($entity = $this->routeMatch->getParameter($entity_type_id)) {
+      if ($entity instanceof OverridesSectionStorageInterface) {
+        return $entity->getDefaultSectionStorage();
+      }
+    }
+  }
+
+}

core/modules/layout_builder/src/Context/LayoutBuilderContextTrait.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace Drupal\layout_builder\Context;
+
+use Drupal\Core\Plugin\Context\ContextInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a wrapper around getting contexts from a section storage object.
+ */
+trait LayoutBuilderContextTrait {
+
+  /**
+   * The context repository.
+   *
+   * @var \Drupal\Core\Plugin\Context\ContextRepositoryInterface
+   */
+  protected $contextRepository;
+
+  /**
+   * Gets the context repository service.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextRepositoryInterface
+   *   The context repository service.
+   */
+  protected function contextRepository() {
+    if (!$this->contextRepository) {
+      $this->contextRepository = \Drupal::service('context.repository');
+    }
+    return $this->contextRepository;
+  }
+
+  /**
+   * Provides all available contexts, both global and section_storage-specific.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextInterface[]
+   *   The array of context objects.
+   */
+  protected function getAvailableContexts(SectionStorageInterface $section_storage) {
+    // Get all globally available contexts that have a defined value.
+    $contexts = array_filter($this->contextRepository()->getAvailableContexts(), function (ContextInterface $context) {
+      return $context->hasContextValue();
+    });
+
+    // Add in the per-section_storage contexts.
+    $contexts += $section_storage->getContexts();
+    return $contexts;
+  }
+
+}

core/modules/layout_builder/src/Controller/AddSectionController.php

@@ -0,0 +1,80 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\DependencyInjection\ClassResolverInterface;
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\RedirectResponse;
+
+/**
+ * Defines a controller to add a new section.
+ *
+ * @internal
+ */
+class AddSectionController implements ContainerInjectionInterface {
+
+  use AjaxHelperTrait;
+  use LayoutRebuildTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * AddSectionController constructor.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
+   *   The class resolver.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, ClassResolverInterface $class_resolver) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->classResolver = $class_resolver;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('class_resolver')
+    );
+  }
+
+  /**
+   * Adds the new section.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   * @param string $plugin_id
+   *   The plugin ID of the layout to add.
+   *
+   * @return \Symfony\Component\HttpFoundation\Response
+   *   The controller response.
+   */
+  public function build(SectionStorageInterface $section_storage, $delta, $plugin_id) {
+    $section_storage->insertSection($delta, new Section($plugin_id));
+
+    $this->layoutTempstoreRepository->set($section_storage);
+
+    if ($this->isAjax()) {
+      return $this->rebuildAndClose($section_storage);
+    }
+    else {
+      $url = $section_storage->getLayoutBuilderUrl();
+      return new RedirectResponse($url->setAbsolute()->toString());
+    }
+  }
+
+}

core/modules/layout_builder/src/Controller/AjaxHelperTrait.php

@@ -0,0 +1,41 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
+
+/**
+ * Provides a helper to determine if the current request is via AJAX.
+ *
+ * @internal
+ *
+ * @todo Move to \Drupal\Core in https://www.drupal.org/node/2896535.
+ */
+trait AjaxHelperTrait {
+
+  /**
+   * Determines if the current request is via AJAX.
+   *
+   * @return bool
+   *   TRUE if the current request is via AJAX, FALSE otherwise.
+   */
+  protected function isAjax() {
+    return in_array($this->getRequestWrapperFormat(), [
+      'drupal_ajax',
+      'drupal_dialog',
+      'drupal_dialog.off_canvas',
+      'drupal_modal',
+    ]);
+  }
+
+  /**
+   * Gets the wrapper format of the current request.
+   *
+   * @string
+   *   The wrapper format.
+   */
+  protected function getRequestWrapperFormat() {
+    return \Drupal::request()->get(MainContentViewSubscriber::WRAPPER_FORMAT);
+  }
+
+}

core/modules/layout_builder/src/Controller/ChooseBlockController.php

@@ -0,0 +1,97 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines a controller to choose a new block.
+ *
+ * @internal
+ */
+class ChooseBlockController implements ContainerInjectionInterface {
+
+  use AjaxHelperTrait;
+  use LayoutBuilderContextTrait;
+
+  /**
+   * The block manager.
+   *
+   * @var \Drupal\Core\Block\BlockManagerInterface
+   */
+  protected $blockManager;
+
+  /**
+   * ChooseBlockController constructor.
+   *
+   * @param \Drupal\Core\Block\BlockManagerInterface $block_manager
+   *   The block manager.
+   */
+  public function __construct(BlockManagerInterface $block_manager) {
+    $this->blockManager = $block_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('plugin.manager.block')
+    );
+  }
+
+  /**
+   * Provides the UI for choosing a new block.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   * @param string $region
+   *   The region the block is going in.
+   *
+   * @return array
+   *   A render array.
+   */
+  public function build(SectionStorageInterface $section_storage, $delta, $region) {
+    $build['#type'] = 'container';
+    $build['#attributes']['class'][] = 'block-categories';
+
+    $definitions = $this->blockManager->getDefinitionsForContexts($this->getAvailableContexts($section_storage));
+    foreach ($this->blockManager->getGroupedDefinitions($definitions) as $category => $blocks) {
+      $build[$category]['#type'] = 'details';
+      $build[$category]['#open'] = TRUE;
+      $build[$category]['#title'] = $category;
+      $build[$category]['links'] = [
+        '#theme' => 'links',
+      ];
+      foreach ($blocks as $block_id => $block) {
+        $link = [
+          'title' => $block['admin_label'],
+          'url' => Url::fromRoute('layout_builder.add_block',
+            [
+              'section_storage_type' => $section_storage->getStorageType(),
+              'section_storage' => $section_storage->getStorageId(),
+              'delta' => $delta,
+              'region' => $region,
+              'plugin_id' => $block_id,
+            ]
+          ),
+        ];
+        if ($this->isAjax()) {
+          $link['attributes']['class'][] = 'use-ajax';
+          $link['attributes']['data-dialog-type'][] = 'dialog';
+          $link['attributes']['data-dialog-renderer'][] = 'off_canvas';
+        }
+        $build[$category]['links']['#links'][] = $link;
+      }
+    }
+    return $build;
+  }
+
+}

core/modules/layout_builder/src/Controller/ChooseSectionController.php

@@ -0,0 +1,105 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Layout\LayoutPluginManagerInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Url;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines a controller to choose a new section.
+ *
+ * @internal
+ */
+class ChooseSectionController implements ContainerInjectionInterface {
+
+  use AjaxHelperTrait;
+  use StringTranslationTrait;
+
+  /**
+   * The layout manager.
+   *
+   * @var \Drupal\Core\Layout\LayoutPluginManagerInterface
+   */
+  protected $layoutManager;
+
+  /**
+   * ChooseSectionController constructor.
+   *
+   * @param \Drupal\Core\Layout\LayoutPluginManagerInterface $layout_manager
+   *   The layout manager.
+   */
+  public function __construct(LayoutPluginManagerInterface $layout_manager) {
+    $this->layoutManager = $layout_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('plugin.manager.core.layout')
+    );
+  }
+
+  /**
+   * Choose a layout plugin to add as a section.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   *
+   * @return array
+   *   The render array.
+   */
+  public function build(SectionStorageInterface $section_storage, $delta) {
+    $output['#title'] = $this->t('Choose a layout');
+
+    $items = [];
+    foreach ($this->layoutManager->getDefinitions() as $plugin_id => $definition) {
+      $layout = $this->layoutManager->createInstance($plugin_id);
+      $item = [
+        '#type' => 'link',
+        '#title' => [
+          $definition->getIcon(60, 80, 1, 3),
+          [
+            '#type' => 'container',
+            '#children' => $definition->getLabel(),
+          ],
+        ],
+        '#url' => Url::fromRoute(
+          $layout instanceof PluginFormInterface ? 'layout_builder.configure_section' : 'layout_builder.add_section',
+          [
+            'section_storage_type' => $section_storage->getStorageType(),
+            'section_storage' => $section_storage->getStorageId(),
+            'delta' => $delta,
+            'plugin_id' => $plugin_id,
+          ]
+        ),
+      ];
+      if ($this->isAjax()) {
+        $item['#attributes']['class'][] = 'use-ajax';
+        $item['#attributes']['data-dialog-type'][] = 'dialog';
+        $item['#attributes']['data-dialog-renderer'][] = 'off_canvas';
+      }
+      $items[] = $item;
+    }
+    $output['layouts'] = [
+      '#theme' => 'item_list',
+      '#items' => $items,
+      '#attributes' => [
+        'class' => [
+          'layout-selection',
+        ],
+      ],
+    ];
+
+    return $output;
+  }
+
+}

core/modules/layout_builder/src/Controller/LayoutBuilderController.php

@@ -0,0 +1,328 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\HttpFoundation\RedirectResponse;
+
+/**
+ * Defines a controller to provide the Layout Builder admin UI.
+ *
+ * @internal
+ */
+class LayoutBuilderController implements ContainerInjectionInterface {
+
+  use LayoutBuilderContextTrait;
+  use StringTranslationTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The messenger service.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * LayoutBuilderController constructor.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->messenger = $messenger;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('messenger')
+    );
+  }
+
+  /**
+   * Provides a title callback.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return string
+   *   The title for the layout page.
+   */
+  public function title(SectionStorageInterface $section_storage) {
+    return $this->t('Edit layout for %label', ['%label' => $section_storage->label()]);
+  }
+
+  /**
+   * Renders the Layout UI.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param bool $is_rebuilding
+   *   (optional) Indicates if the layout is rebuilding, defaults to FALSE.
+   *
+   * @return array
+   *   A render array.
+   */
+  public function layout(SectionStorageInterface $section_storage, $is_rebuilding = FALSE) {
+    $this->prepareLayout($section_storage, $is_rebuilding);
+
+    $output = [];
+    $count = 0;
+    for ($i = 0; $i < $section_storage->count(); $i++) {
+      $output[] = $this->buildAddSectionLink($section_storage, $count);
+      $output[] = $this->buildAdministrativeSection($section_storage, $count);
+      $count++;
+    }
+    $output[] = $this->buildAddSectionLink($section_storage, $count);
+    $output['#attached']['library'][] = 'layout_builder/drupal.layout_builder';
+    $output['#type'] = 'container';
+    $output['#attributes']['id'] = 'layout-builder';
+    // Mark this UI as uncacheable.
+    $output['#cache']['max-age'] = 0;
+    return $output;
+  }
+
+  /**
+   * Prepares a layout for use in the UI.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param bool $is_rebuilding
+   *   Indicates if the layout is rebuilding.
+   */
+  protected function prepareLayout(SectionStorageInterface $section_storage, $is_rebuilding) {
+    // Only add sections if the layout is new and empty.
+    if (!$is_rebuilding && $section_storage->count() === 0) {
+      $sections = [];
+      // If this is an empty override, copy the sections from the corresponding
+      // default.
+      if ($section_storage instanceof OverridesSectionStorageInterface) {
+        $sections = $section_storage->getDefaultSectionStorage()->getSections();
+      }
+
+      // For an empty layout, begin with a single section of one column.
+      if (!$sections) {
+        $sections[] = new Section('layout_onecol');
+      }
+
+      foreach ($sections as $section) {
+        $section_storage->appendSection($section);
+      }
+      $this->layoutTempstoreRepository->set($section_storage);
+    }
+  }
+
+  /**
+   * Builds a link to add a new section at a given delta.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section to splice.
+   *
+   * @return array
+   *   A render array for a link.
+   */
+  protected function buildAddSectionLink(SectionStorageInterface $section_storage, $delta) {
+    $storage_type = $section_storage->getStorageType();
+    $storage_id = $section_storage->getStorageId();
+    return [
+      'link' => [
+        '#type' => 'link',
+        '#title' => $this->t('Add Section'),
+        '#url' => Url::fromRoute('layout_builder.choose_section',
+          [
+            'section_storage_type' => $storage_type,
+            'section_storage' => $storage_id,
+            'delta' => $delta,
+          ],
+          [
+            'attributes' => [
+              'class' => ['use-ajax'],
+              'data-dialog-type' => 'dialog',
+              'data-dialog-renderer' => 'off_canvas',
+            ],
+          ]
+        ),
+      ],
+      '#type' => 'container',
+      '#attributes' => [
+        'class' => ['add-section'],
+      ],
+    ];
+  }
+
+  /**
+   * Builds the render array for the layout section while editing.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta
+   *   The delta of the section.
+   *
+   * @return array
+   *   The render array for a given section.
+   */
+  protected function buildAdministrativeSection(SectionStorageInterface $section_storage, $delta) {
+    $storage_type = $section_storage->getStorageType();
+    $storage_id = $section_storage->getStorageId();
+    $section = $section_storage->getSection($delta);
+
+    $layout = $section->getLayout();
+    $build = $section->toRenderArray($this->getAvailableContexts($section_storage), TRUE);
+    $layout_definition = $layout->getPluginDefinition();
+
+    foreach ($layout_definition->getRegions() as $region => $info) {
+      if (!empty($build[$region])) {
+        foreach ($build[$region] as $uuid => $block) {
+          $build[$region][$uuid]['#attributes']['class'][] = 'draggable';
+          $build[$region][$uuid]['#attributes']['data-layout-block-uuid'] = $uuid;
+          $build[$region][$uuid]['#contextual_links'] = [
+            'layout_builder_block' => [
+              'route_parameters' => [
+                'section_storage_type' => $storage_type,
+                'section_storage' => $storage_id,
+                'delta' => $delta,
+                'region' => $region,
+                'uuid' => $uuid,
+              ],
+            ],
+          ];
+        }
+      }
+
+      $build[$region]['layout_builder_add_block']['link'] = [
+        '#type' => 'link',
+        '#title' => $this->t('Add Block'),
+        '#url' => Url::fromRoute('layout_builder.choose_block',
+          [
+            'section_storage_type' => $storage_type,
+            'section_storage' => $storage_id,
+            'delta' => $delta,
+            'region' => $region,
+          ],
+          [
+            'attributes' => [
+              'class' => ['use-ajax'],
+              'data-dialog-type' => 'dialog',
+              'data-dialog-renderer' => 'off_canvas',
+            ],
+          ]
+        ),
+      ];
+      $build[$region]['layout_builder_add_block']['#type'] = 'container';
+      $build[$region]['layout_builder_add_block']['#attributes'] = ['class' => ['add-block']];
+      $build[$region]['layout_builder_add_block']['#weight'] = 1000;
+      $build[$region]['#attributes']['data-region'] = $region;
+      $build[$region]['#attributes']['class'][] = 'layout-builder--layout__region';
+    }
+
+    $build['#attributes']['data-layout-update-url'] = Url::fromRoute('layout_builder.move_block', [
+      'section_storage_type' => $storage_type,
+      'section_storage' => $storage_id,
+    ])->toString();
+    $build['#attributes']['data-layout-delta'] = $delta;
+    $build['#attributes']['class'][] = 'layout-builder--layout';
+
+    return [
+      '#type' => 'container',
+      '#attributes' => [
+        'class' => ['layout-section'],
+      ],
+      'configure' => [
+        '#type' => 'link',
+        '#title' => $this->t('Configure section'),
+        '#access' => $layout instanceof PluginFormInterface,
+        '#url' => Url::fromRoute('layout_builder.configure_section', [
+          'section_storage_type' => $storage_type,
+          'section_storage' => $storage_id,
+          'delta' => $delta,
+        ]),
+        '#attributes' => [
+          'class' => ['use-ajax', 'configure-section'],
+          'data-dialog-type' => 'dialog',
+          'data-dialog-renderer' => 'off_canvas',
+        ],
+      ],
+      'remove' => [
+        '#type' => 'link',
+        '#title' => $this->t('Remove section'),
+        '#url' => Url::fromRoute('layout_builder.remove_section', [
+          'section_storage_type' => $storage_type,
+          'section_storage' => $storage_id,
+          'delta' => $delta,
+        ]),
+        '#attributes' => [
+          'class' => ['use-ajax', 'remove-section'],
+          'data-dialog-type' => 'dialog',
+          'data-dialog-renderer' => 'off_canvas',
+        ],
+      ],
+      'layout-section' => $build,
+    ];
+  }
+
+  /**
+   * Saves the layout.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Symfony\Component\HttpFoundation\RedirectResponse
+   *   A redirect response.
+   */
+  public function saveLayout(SectionStorageInterface $section_storage) {
+    $section_storage->save();
+    $this->layoutTempstoreRepository->delete($section_storage);
+
+    if ($section_storage instanceof OverridesSectionStorageInterface) {
+      $this->messenger->addMessage($this->t('The layout override has been saved.'));
+    }
+    else {
+      $this->messenger->addMessage($this->t('The layout has been saved.'));
+    }
+
+    return new RedirectResponse($section_storage->getRedirectUrl()->setAbsolute()->toString());
+  }
+
+  /**
+   * Cancels the layout.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Symfony\Component\HttpFoundation\RedirectResponse
+   *   A redirect response.
+   */
+  public function cancelLayout(SectionStorageInterface $section_storage) {
+    $this->layoutTempstoreRepository->delete($section_storage);
+
+    $this->messenger->addMessage($this->t('The changes to the layout have been discarded.'));
+
+    return new RedirectResponse($section_storage->getRedirectUrl()->setAbsolute()->toString());
+  }
+
+}

core/modules/layout_builder/src/Controller/LayoutRebuildTrait.php

@@ -0,0 +1,58 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\CloseDialogCommand;
+use Drupal\Core\Ajax\ReplaceCommand;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides AJAX responses to rebuild the Layout Builder.
+ *
+ * @internal
+ */
+trait LayoutRebuildTrait {
+
+  /**
+   * The class resolver.
+   *
+   * @var \Drupal\Core\DependencyInjection\ClassResolverInterface
+   */
+  protected $classResolver;
+
+  /**
+   * Rebuilds the layout.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   An AJAX response to either rebuild the layout and close the dialog, or
+   *   reload the page.
+   */
+  protected function rebuildAndClose(SectionStorageInterface $section_storage) {
+    $response = $this->rebuildLayout($section_storage);
+    $response->addCommand(new CloseDialogCommand('#drupal-off-canvas'));
+    return $response;
+  }
+
+  /**
+   * Rebuilds the layout.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   An AJAX response to either rebuild the layout and close the dialog, or
+   *   reload the page.
+   */
+  protected function rebuildLayout(SectionStorageInterface $section_storage) {
+    $response = new AjaxResponse();
+    $layout_controller = $this->classResolver->getInstanceFromDefinition(LayoutBuilderController::class);
+    $layout = $layout_controller->layout($section_storage, TRUE);
+    $response->addCommand(new ReplaceCommand('#layout-builder', $layout));
+    return $response;
+  }
+
+}

core/modules/layout_builder/src/Controller/MoveBlockController.php

@@ -0,0 +1,95 @@
+<?php
+
+namespace Drupal\layout_builder\Controller;
+
+use Drupal\Core\DependencyInjection\ClassResolverInterface;
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Defines a controller to move a block.
+ *
+ * @internal
+ */
+class MoveBlockController implements ContainerInjectionInterface {
+
+  use LayoutRebuildTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * LayoutController constructor.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
+   *   The class resolver.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, ClassResolverInterface $class_resolver) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->classResolver = $class_resolver;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('class_resolver')
+    );
+  }
+
+  /**
+   * Moves a block to another region.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param int $delta_from
+   *   The delta of the original section.
+   * @param int $delta_to
+   *   The delta of the destination section.
+   * @param string $region_to
+   *   The new region for this block.
+   * @param string $block_uuid
+   *   The UUID for this block.
+   * @param string|null $preceding_block_uuid
+   *   (optional) If provided, the UUID of the block to insert this block after.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   An AJAX response.
+   */
+  public function build(SectionStorageInterface $section_storage, $delta_from, $delta_to, $region_to, $block_uuid, $preceding_block_uuid = NULL) {
+    $section = $section_storage->getSection($delta_from);
+
+    $component = $section->getComponent($block_uuid);
+    $section->removeComponent($block_uuid);
+
+    // If the block is moving from one section to another, update the original
+    // section and load the new one.
+    if ($delta_from !== $delta_to) {
+      $section = $section_storage->getSection($delta_to);
+    }
+
+    // If a preceding block was specified, insert after that. Otherwise add the
+    // block to the front.
+    $component->setRegion($region_to);
+    if (isset($preceding_block_uuid)) {
+      $section->insertAfterComponent($preceding_block_uuid, $component);
+    }
+    else {
+      $section->insertComponent(0, $component);
+    }
+
+    $this->layoutTempstoreRepository->set($section_storage);
+    return $this->rebuildLayout($section_storage);
+  }
+
+}

core/modules/layout_builder/src/DefaultsSectionStorageInterface.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Defines an interface for an object that stores layout sections for defaults.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+interface DefaultsSectionStorageInterface extends SectionStorageInterface {
+
+  /**
+   * Determines if the defaults allow custom overrides.
+   *
+   * @return bool
+   *   TRUE if custom overrides are allowed, FALSE otherwise.
+   */
+  public function isOverridable();
+
+  /**
+   * Sets the defaults to allow or disallow overrides.
+   *
+   * @param bool $overridable
+   *   TRUE if the display should allow overrides, FALSE otherwise.
+   *
+   * @return $this
+   */
+  public function setOverridable($overridable = TRUE);
+
+}

core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplay.php

@@ -0,0 +1,332 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+use Drupal\Component\Plugin\Definition\PluginDefinitionInterface;
+use Drupal\Component\Plugin\DependentPluginInterface;
+use Drupal\Component\Plugin\PluginInspectionInterface;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Entity\Entity\EntityViewDisplay as BaseEntityViewDisplay;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Plugin\Context\Context;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Definition\DependentPluginDefinitionInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\layout_builder\SectionStorage\SectionStorageTrait;
+
+/**
+ * Provides an entity view display entity that has a layout.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class LayoutBuilderEntityViewDisplay extends BaseEntityViewDisplay implements LayoutEntityDisplayInterface {
+
+  use SectionStorageTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isOverridable() {
+    return $this->getThirdPartySetting('layout_builder', 'allow_custom', FALSE);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setOverridable($overridable = TRUE) {
+    $this->setThirdPartySetting('layout_builder', 'allow_custom', $overridable);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSections() {
+    return $this->getThirdPartySetting('layout_builder', 'sections', []);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setSections(array $sections) {
+    $this->setThirdPartySetting('layout_builder', 'sections', array_values($sections));
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function preSave(EntityStorageInterface $storage) {
+    parent::preSave($storage);
+
+    $original_value = isset($this->original) ? $this->original->isOverridable() : FALSE;
+    $new_value = $this->isOverridable();
+    if ($original_value !== $new_value) {
+      $entity_type_id = $this->getTargetEntityTypeId();
+      $bundle = $this->getTargetBundle();
+
+      if ($new_value) {
+        $this->addSectionField($entity_type_id, $bundle, 'layout_builder__layout');
+      }
+      elseif ($field = FieldConfig::loadByName($entity_type_id, $bundle, 'layout_builder__layout')) {
+        $field->delete();
+      }
+    }
+  }
+
+  /**
+   * Adds a layout section field to a given bundle.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle
+   *   The bundle.
+   * @param string $field_name
+   *   The name for the layout section field.
+   */
+  protected function addSectionField($entity_type_id, $bundle, $field_name) {
+    $field = FieldConfig::loadByName($entity_type_id, $bundle, $field_name);
+    if (!$field) {
+      $field_storage = FieldStorageConfig::loadByName($entity_type_id, $field_name);
+      if (!$field_storage) {
+        $field_storage = FieldStorageConfig::create([
+          'entity_type' => $entity_type_id,
+          'field_name' => $field_name,
+          'type' => 'layout_section',
+          'locked' => TRUE,
+        ]);
+        $field_storage->save();
+      }
+
+      $field = FieldConfig::create([
+        'field_storage' => $field_storage,
+        'bundle' => $bundle,
+        'label' => t('Layout'),
+      ]);
+      $field->save();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getDefaultRegion() {
+    if ($this->hasSection(0)) {
+      return $this->getSection(0)->getDefaultRegion();
+    }
+
+    return parent::getDefaultRegion();
+  }
+
+  /**
+   * Wraps the context repository service.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextRepositoryInterface
+   *   The context repository service.
+   */
+  protected function contextRepository() {
+    return \Drupal::service('context.repository');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildMultiple(array $entities) {
+    $build_list = parent::buildMultiple($entities);
+
+    foreach ($entities as $id => $entity) {
+      $sections = $this->getRuntimeSections($entity);
+      if ($sections) {
+        foreach ($build_list[$id] as $name => $build_part) {
+          $field_definition = $this->getFieldDefinition($name);
+          if ($field_definition && $field_definition->isDisplayConfigurable($this->displayContext)) {
+            unset($build_list[$id][$name]);
+          }
+        }
+
+        // Bypass ::getContexts() in order to use the runtime entity, not a
+        // sample entity.
+        $contexts = $this->contextRepository()->getAvailableContexts();
+        // @todo Use EntityContextDefinition after resolving
+        //   https://www.drupal.org/node/2932462.
+        $contexts['layout_builder.entity'] = new Context(new ContextDefinition("entity:{$entity->getEntityTypeId()}", new TranslatableMarkup('@entity being viewed', ['@entity' => $entity->getEntityType()->getLabel()])), $entity);
+        foreach ($sections as $delta => $section) {
+          $build_list[$id]['_layout_builder'][$delta] = $section->toRenderArray($contexts);
+        }
+      }
+    }
+
+    return $build_list;
+  }
+
+  /**
+   * Gets the runtime sections for a given entity.
+   *
+   * @param \Drupal\Core\Entity\FieldableEntityInterface $entity
+   *   The entity.
+   *
+   * @return \Drupal\layout_builder\Section[]
+   *   The sections.
+   */
+  protected function getRuntimeSections(FieldableEntityInterface $entity) {
+    if ($this->isOverridable() && !$entity->get('layout_builder__layout')->isEmpty()) {
+      return $entity->get('layout_builder__layout')->getSections();
+    }
+
+    return $this->getSections();
+  }
+
+  /**
+   * {@inheritdoc}
+   *
+   * @todo Move this upstream in https://www.drupal.org/node/2939931.
+   */
+  public function label() {
+    $bundle_info = \Drupal::service('entity_type.bundle.info')->getBundleInfo($this->getTargetEntityTypeId());
+    $bundle_label = $bundle_info[$this->getTargetBundle()]['label'];
+    $target_entity_type = $this->entityTypeManager()->getDefinition($this->getTargetEntityTypeId());
+    return new TranslatableMarkup('@bundle @label', ['@bundle' => $bundle_label, '@label' => $target_entity_type->getPluralLabel()]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function calculateDependencies() {
+    parent::calculateDependencies();
+
+    foreach ($this->getSections() as $delta => $section) {
+      $this->calculatePluginDependencies($section->getLayout());
+      foreach ($section->getComponents() as $uuid => $component) {
+        $this->calculatePluginDependencies($component->getPlugin());
+      }
+    }
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function onDependencyRemoval(array $dependencies) {
+    $changed = parent::onDependencyRemoval($dependencies);
+
+    // Loop through all sections and determine if the removed dependencies are
+    // used by their layout plugins.
+    foreach ($this->getSections() as $delta => $section) {
+      $layout_dependencies = $this->getPluginDependencies($section->getLayout());
+      $layout_removed_dependencies = $this->getPluginRemovedDependencies($layout_dependencies, $dependencies);
+      if ($layout_removed_dependencies) {
+        // @todo Allow the plugins to react to their dependency removal in
+        //   https://www.drupal.org/project/drupal/issues/2579743.
+        $this->removeSection($delta);
+        $changed = TRUE;
+      }
+      // If the section is not removed, loop through all components.
+      else {
+        foreach ($section->getComponents() as $uuid => $component) {
+          $plugin_dependencies = $this->getPluginDependencies($component->getPlugin());
+          $component_removed_dependencies = $this->getPluginRemovedDependencies($plugin_dependencies, $dependencies);
+          if ($component_removed_dependencies) {
+            // @todo Allow the plugins to react to their dependency removal in
+            //   https://www.drupal.org/project/drupal/issues/2579743.
+            $section->removeComponent($uuid);
+            $changed = TRUE;
+          }
+        }
+      }
+    }
+    return $changed;
+  }
+
+  /**
+   * Calculates and returns dependencies of a specific plugin instance.
+   *
+   * @param \Drupal\Component\Plugin\PluginInspectionInterface $instance
+   *   The plugin instance.
+   *
+   * @return array
+   *   An array of dependencies keyed by the type of dependency.
+   *
+   * @todo Replace this in https://www.drupal.org/project/drupal/issues/2939925.
+   */
+  protected function getPluginDependencies(PluginInspectionInterface $instance) {
+    $dependencies = [];
+    $definition = $instance->getPluginDefinition();
+    if ($definition instanceof PluginDefinitionInterface) {
+      $dependencies['module'][] = $definition->getProvider();
+      if ($definition instanceof DependentPluginDefinitionInterface && $config_dependencies = $definition->getConfigDependencies()) {
+        $dependencies = NestedArray::mergeDeep($dependencies, $config_dependencies);
+      }
+    }
+    elseif (is_array($definition)) {
+      $dependencies['module'][] = $definition['provider'];
+      // Plugins can declare additional dependencies in their definition.
+      if (isset($definition['config_dependencies'])) {
+        $dependencies = NestedArray::mergeDeep($dependencies, $definition['config_dependencies']);
+      }
+    }
+
+    // If a plugin is dependent, calculate its dependencies.
+    if ($instance instanceof DependentPluginInterface && $plugin_dependencies = $instance->calculateDependencies()) {
+      $dependencies = NestedArray::mergeDeep($dependencies, $plugin_dependencies);
+    }
+    return $dependencies;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setComponent($name, array $options = []) {
+    parent::setComponent($name, $options);
+
+    // @todo Remove workaround for EntityViewBuilder::getSingleFieldDisplay() in
+    //   https://www.drupal.org/project/drupal/issues/2936464.
+    if ($this->getMode() === static::CUSTOM_MODE) {
+      return $this;
+    }
+
+    // Retrieve the updated options after the parent:: call.
+    $options = $this->content[$name];
+    // Provide backwards compatibility by converting to a section component.
+    $field_definition = $this->getFieldDefinition($name);
+    if ($field_definition && $field_definition->isDisplayConfigurable('view') && isset($options['type'])) {
+      $configuration = [];
+      $configuration['id'] = 'field_block:' . $this->getTargetEntityTypeId() . ':' . $this->getTargetBundle() . ':' . $name;
+      $configuration['label_display'] = FALSE;
+      $keys = array_flip(['type', 'label', 'settings', 'third_party_settings']);
+      $configuration['formatter'] = array_intersect_key($options, $keys);
+      $configuration['context_mapping']['entity'] = 'layout_builder.entity';
+
+      $section = $this->getDefaultSection();
+      $region = isset($options['region']) ? $options['region'] : $section->getDefaultRegion();
+      $new_component = (new SectionComponent(\Drupal::service('uuid')->generate(), $region, $configuration));
+      $section->appendComponent($new_component);
+    }
+    return $this;
+  }
+
+  /**
+   * Gets a default section.
+   *
+   * @return \Drupal\layout_builder\Section
+   *   The default section.
+   */
+  protected function getDefaultSection() {
+    // If no section exists, append a new one.
+    if (!$this->hasSection(0)) {
+      $this->appendSection(new Section('layout_onecol'));
+    }
+
+    // Return the first section.
+    return $this->getSection(0);
+  }
+
+}

core/modules/layout_builder/src/Entity/LayoutBuilderEntityViewDisplayStorage.php

@@ -0,0 +1,60 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+use Drupal\Core\Config\Entity\ConfigEntityStorage;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionComponent;
+
+/**
+ * Provides storage for entity view display entities that have layouts.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class LayoutBuilderEntityViewDisplayStorage extends ConfigEntityStorage {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function mapToStorageRecord(EntityInterface $entity) {
+    $record = parent::mapToStorageRecord($entity);
+
+    if (!empty($record['third_party_settings']['layout_builder']['sections'])) {
+      $record['third_party_settings']['layout_builder']['sections'] = array_map(function (Section $section) {
+        return $section->toArray();
+      }, $record['third_party_settings']['layout_builder']['sections']);
+    }
+    return $record;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function mapFromStorageRecords(array $records) {
+    foreach ($records as $id => &$record) {
+      if (!empty($record['third_party_settings']['layout_builder']['sections'])) {
+        $sections = &$record['third_party_settings']['layout_builder']['sections'];
+        foreach ($sections as $section_delta => $section) {
+          $sections[$section_delta] = new Section(
+            $section['layout_id'],
+            $section['layout_settings'],
+            array_map(function (array $component) {
+              return (new SectionComponent(
+                $component['uuid'],
+                $component['region'],
+                $component['configuration'],
+                $component['additional']
+              ))->setWeight($component['weight']);
+            }, $section['components'])
+          );
+        }
+      }
+    }
+    return parent::mapFromStorageRecords($records);
+  }
+
+}

core/modules/layout_builder/src/Entity/LayoutBuilderSampleEntityGenerator.php

@@ -0,0 +1,91 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+use Drupal\Core\Entity\ContentEntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\TempStore\SharedTempStoreFactory;
+
+/**
+ * Generates a sample entity for use by the Layout Builder.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class LayoutBuilderSampleEntityGenerator {
+
+  /**
+   * The shared tempstore factory.
+   *
+   * @var \Drupal\Core\TempStore\SharedTempStoreFactory
+   */
+  protected $tempStoreFactory;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * LayoutBuilderSampleEntityGenerator constructor.
+   *
+   * @param \Drupal\Core\TempStore\SharedTempStoreFactory $temp_store_factory
+   *   The tempstore factory.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   */
+  public function __construct(SharedTempStoreFactory $temp_store_factory, EntityTypeManagerInterface $entity_type_manager) {
+    $this->tempStoreFactory = $temp_store_factory;
+    $this->entityTypeManager = $entity_type_manager;
+  }
+
+  /**
+   * Gets a sample entity for a given entity type and bundle.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle_id
+   *   The bundle ID.
+   *
+   * @return \Drupal\Core\Entity\EntityInterface
+   *   An entity.
+   */
+  public function get($entity_type_id, $bundle_id) {
+    $tempstore = $this->tempStoreFactory->get('layout_builder.sample_entity');
+    if ($entity = $tempstore->get("$entity_type_id.$bundle_id")) {
+      return $entity;
+    }
+
+    $entity_storage = $this->entityTypeManager->getStorage($entity_type_id);
+    if (!$entity_storage instanceof ContentEntityStorageInterface) {
+      throw new \InvalidArgumentException(sprintf('The "%s" entity storage is not supported', $entity_type_id));
+    }
+
+    $entity = $entity_storage->createWithSampleValues($bundle_id);
+    // Mark the sample entity as being a preview.
+    $entity->in_preview = TRUE;
+    $tempstore->set("$entity_type_id.$bundle_id", $entity);
+    return $entity;
+  }
+
+  /**
+   * Deletes a sample entity for a given entity type and bundle.
+   *
+   * @param string $entity_type_id
+   *   The entity type ID.
+   * @param string $bundle_id
+   *   The bundle ID.
+   *
+   * @return $this
+   */
+  public function delete($entity_type_id, $bundle_id) {
+    $tempstore = $this->tempStoreFactory->get('layout_builder.sample_entity');
+    $tempstore->delete("$entity_type_id.$bundle_id");
+    return $this;
+  }
+
+}

core/modules/layout_builder/src/Entity/LayoutEntityDisplayInterface.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace Drupal\layout_builder\Entity;
+
+use Drupal\Core\Entity\Display\EntityDisplayInterface;
+use Drupal\layout_builder\SectionListInterface;
+
+/**
+ * Provides an interface for entity displays that have layout.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+interface LayoutEntityDisplayInterface extends EntityDisplayInterface, SectionListInterface {
+
+  /**
+   * Determines if the display allows custom overrides.
+   *
+   * @return bool
+   *   TRUE if custom overrides are allowed, FALSE otherwise.
+   */
+  public function isOverridable();
+
+  /**
+   * Sets the display to allow or disallow overrides.
+   *
+   * @param bool $overridable
+   *   TRUE if the display should allow overrides, FALSE otherwise.
+   *
+   * @return $this
+   */
+  public function setOverridable($overridable = TRUE);
+
+}

core/modules/layout_builder/src/Event/SectionComponentBuildRenderArrayEvent.php

@@ -0,0 +1,138 @@
+<?php
+
+namespace Drupal\layout_builder\Event;
+
+use Drupal\Core\Cache\CacheableResponseTrait;
+use Drupal\layout_builder\SectionComponent;
+use Symfony\Component\EventDispatcher\Event;
+
+/**
+ * Event fired when a section component's render array is being built.
+ *
+ * Subscribers to this event should manipulate the cacheability object and the
+ * build array in this event.
+ *
+ * @see \Drupal\layout_builder\LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class SectionComponentBuildRenderArrayEvent extends Event {
+
+  use CacheableResponseTrait;
+
+  /**
+   * The section component whose render array is being built.
+   *
+   * @var \Drupal\layout_builder\SectionComponent
+   */
+  protected $component;
+
+  /**
+   * The available contexts.
+   *
+   * @var \Drupal\Core\Plugin\Context\ContextInterface[]
+   */
+  protected $contexts;
+
+  /**
+   * The plugin for the section component being built.
+   *
+   * @var \Drupal\Component\Plugin\PluginInspectionInterface
+   */
+  protected $plugin;
+
+  /**
+   * Whether the component is in preview mode or not.
+   *
+   * @var bool
+   */
+  protected $inPreview;
+
+  /**
+   * The render array built by the event subscribers.
+   *
+   * @var array
+   */
+  protected $build = [];
+
+  /**
+   * Creates a new SectionComponentBuildRenderArrayEvent object.
+   *
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The section component whose render array is being built.
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   The available contexts.
+   * @param bool $in_preview
+   *   (optional) Whether the component is in preview mode or not.
+   */
+  public function __construct(SectionComponent $component, array $contexts, $in_preview = FALSE) {
+    $this->component = $component;
+    $this->contexts = $contexts;
+    $this->plugin = $component->getPlugin($contexts);
+    $this->inPreview = $in_preview;
+  }
+
+  /**
+   * Get the section component whose render array is being built.
+   *
+   * @return \Drupal\layout_builder\SectionComponent
+   *   The section component whose render array is being built.
+   */
+  public function getComponent() {
+    return $this->component;
+  }
+
+  /**
+   * Get the available contexts.
+   *
+   * @return array|\Drupal\Core\Plugin\Context\ContextInterface[]
+   *   The available contexts.
+   */
+  public function getContexts() {
+    return $this->contexts;
+  }
+
+  /**
+   * Get the plugin for the section component being built.
+   *
+   * @return \Drupal\Component\Plugin\PluginInspectionInterface
+   *   The plugin for the section component being built.
+   */
+  public function getPlugin() {
+    return $this->plugin;
+  }
+
+  /**
+   * Determine if the component is in preview mode.
+   *
+   * @return bool
+   *   Whether the component is in preview mode or not.
+   */
+  public function inPreview() {
+    return $this->inPreview;
+  }
+
+  /**
+   * Get the render array in its current state.
+   *
+   * @return array
+   *   The render array built by the event subscribers.
+   */
+  public function getBuild() {
+    return $this->build;
+  }
+
+  /**
+   * Set the render array.
+   *
+   * @param array $build
+   *   A render array.
+   */
+  public function setBuild(array $build) {
+    $this->build = $build;
+  }
+
+}

core/modules/layout_builder/src/EventSubscriber/BlockComponentRenderArray.php

@@ -0,0 +1,85 @@
+<?php
+
+namespace Drupal\layout_builder\EventSubscriber;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent;
+use Drupal\layout_builder\LayoutBuilderEvents;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Builds render arrays and handles access for all block components.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class BlockComponentRenderArray implements EventSubscriberInterface {
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * Creates a BlockComponentRenderArray object.
+   *
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current user.
+   */
+  public function __construct(AccountInterface $current_user) {
+    $this->currentUser = $current_user;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    $events[LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY] = ['onBuildRender', 100];
+    return $events;
+  }
+
+  /**
+   * Builds render arrays for block plugins and sets it on the event.
+   *
+   * @param \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent $event
+   *   The section component render event.
+   */
+  public function onBuildRender(SectionComponentBuildRenderArrayEvent $event) {
+    $block = $event->getPlugin();
+    if (!$block instanceof BlockPluginInterface) {
+      return;
+    }
+
+    // Only check access if the component is not being previewed.
+    if ($event->inPreview()) {
+      $access = AccessResult::allowed()->setCacheMaxAge(0);
+    }
+    else {
+      $access = $block->access($this->currentUser, TRUE);
+    }
+
+    $event->addCacheableDependency($access);
+    if ($access->isAllowed()) {
+      $event->addCacheableDependency($block);
+
+      $build = [
+        // @todo Move this to BlockBase in https://www.drupal.org/node/2931040.
+        '#theme' => 'block',
+        '#configuration' => $block->getConfiguration(),
+        '#plugin_id' => $block->getPluginId(),
+        '#base_plugin_id' => $block->getBaseId(),
+        '#derivative_plugin_id' => $block->getDerivativeId(),
+        '#weight' => $event->getComponent()->getWeight(),
+        'content' => $block->build(),
+      ];
+      $event->setBuild($build);
+    }
+  }
+
+}

core/modules/layout_builder/src/Field/LayoutSectionItemList.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace Drupal\layout_builder\Field;
+
+use Drupal\Core\Field\FieldItemList;
+use Drupal\layout_builder\SectionListInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageTrait;
+
+/**
+ * Defines a item list class for layout section fields.
+ *
+ * @internal
+ *
+ * @see \Drupal\layout_builder\Plugin\Field\FieldType\LayoutSectionItem
+ */
+class LayoutSectionItemList extends FieldItemList implements SectionListInterface {
+
+  use SectionStorageTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSections() {
+    $sections = [];
+    /** @var \Drupal\layout_builder\Plugin\Field\FieldType\LayoutSectionItem $item */
+    foreach ($this->list as $delta => $item) {
+      $sections[$delta] = $item->section;
+    }
+    return $sections;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setSections(array $sections) {
+    $this->list = [];
+    $sections = array_values($sections);
+    /** @var \Drupal\layout_builder\Plugin\Field\FieldType\LayoutSectionItem $item */
+    foreach ($sections as $section) {
+      $item = $this->appendItem();
+      $item->section = $section;
+    }
+
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __wakeup() {
+    // Ensure the entity is updated with the latest value.
+    $this->getEntity()->set($this->getName(), $this->getValue());
+  }
+
+}

core/modules/layout_builder/src/Form/AddBlockForm.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a form to add a block.
+ *
+ * @internal
+ */
+class AddBlockForm extends ConfigureBlockFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_add_block';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function submitLabel() {
+    return $this->t('Add Block');
+  }
+
+  /**
+   * Builds the form for the block.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage being configured.
+   * @param int $delta
+   *   The delta of the section.
+   * @param string $region
+   *   The region of the block.
+   * @param string|null $plugin_id
+   *   The plugin ID of the block to add.
+   *
+   * @return array
+   *   The form array.
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL, $delta = NULL, $region = NULL, $plugin_id = NULL) {
+    // Only generate a new component once per form submission.
+    if (!$component = $form_state->getTemporaryValue('layout_builder__component')) {
+      $component = new SectionComponent($this->uuidGenerator->generate(), $region, ['id' => $plugin_id]);
+      $section_storage->getSection($delta)->appendComponent($component);
+      $form_state->setTemporaryValue('layout_builder__component', $component);
+    }
+    return $this->doBuildForm($form, $form_state, $section_storage, $delta, $component);
+  }
+
+}

core/modules/layout_builder/src/Form/AjaxFormHelperTrait.php

@@ -0,0 +1,61 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\ReplaceCommand;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\Controller\AjaxHelperTrait;
+
+/**
+ * Provides a helper to for submitting an AJAX form.
+ *
+ * @internal
+ *
+ * @todo Move to \Drupal\Core in https://www.drupal.org/node/2896535.
+ */
+trait AjaxFormHelperTrait {
+
+  use AjaxHelperTrait;
+
+  /**
+   * Submit form dialog #ajax callback.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   An AJAX response that display validation error messages or represents a
+   *   successful submission.
+   */
+  public function ajaxSubmit(array &$form, FormStateInterface $form_state) {
+    if ($form_state->hasAnyErrors()) {
+      $form['status_messages'] = [
+        '#type' => 'status_messages',
+        '#weight' => -1000,
+      ];
+      $response = new AjaxResponse();
+      $response->addCommand(new ReplaceCommand('[data-drupal-selector="' . $form['#attributes']['data-drupal-selector'] . '"]', $form));
+    }
+    else {
+      $response = $this->successfulAjaxSubmit($form, $form_state);
+    }
+    return $response;
+  }
+
+  /**
+   * Allows the form to respond to a successful AJAX submission.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   *
+   * @return \Drupal\Core\Ajax\AjaxResponse
+   *   An AJAX response.
+   */
+  abstract protected function successfulAjaxSubmit(array $form, FormStateInterface $form_state);
+
+}

core/modules/layout_builder/src/Form/ConfigureBlockFormBase.php

@@ -0,0 +1,244 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Component\Uuid\UuidInterface;
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\DependencyInjection\ClassResolverInterface;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\SubformState;
+use Drupal\Core\Plugin\Context\ContextRepositoryInterface;
+use Drupal\Core\Plugin\ContextAwarePluginAssignmentTrait;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Plugin\PluginFormFactoryInterface;
+use Drupal\Core\Plugin\PluginWithFormsInterface;
+use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\Controller\LayoutRebuildTrait;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a base form for configuring a block.
+ *
+ * @internal
+ */
+abstract class ConfigureBlockFormBase extends FormBase {
+
+  use AjaxFormHelperTrait;
+  use ContextAwarePluginAssignmentTrait;
+  use LayoutBuilderContextTrait;
+  use LayoutRebuildTrait;
+
+  /**
+   * The plugin being configured.
+   *
+   * @var \Drupal\Core\Block\BlockPluginInterface
+   */
+  protected $block;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The block manager.
+   *
+   * @var \Drupal\Core\Block\BlockManagerInterface
+   */
+  protected $blockManager;
+
+  /**
+   * The UUID generator.
+   *
+   * @var \Drupal\Component\Uuid\UuidInterface
+   */
+  protected $uuidGenerator;
+
+  /**
+   * The plugin form manager.
+   *
+   * @var \Drupal\Core\Plugin\PluginFormFactoryInterface
+   */
+  protected $pluginFormFactory;
+
+  /**
+   * The field delta.
+   *
+   * @var int
+   */
+  protected $delta;
+
+  /**
+   * The current region.
+   *
+   * @var string
+   */
+  protected $region;
+
+  /**
+   * The UUID of the component.
+   *
+   * @var string
+   */
+  protected $uuid;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new block form.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Plugin\Context\ContextRepositoryInterface $context_repository
+   *   The context repository.
+   * @param \Drupal\Core\Block\BlockManagerInterface $block_manager
+   *   The block manager.
+   * @param \Drupal\Component\Uuid\UuidInterface $uuid
+   *   The UUID generator.
+   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
+   *   The class resolver.
+   * @param \Drupal\Core\Plugin\PluginFormFactoryInterface $plugin_form_manager
+   *   The plugin form manager.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, ContextRepositoryInterface $context_repository, BlockManagerInterface $block_manager, UuidInterface $uuid, ClassResolverInterface $class_resolver, PluginFormFactoryInterface $plugin_form_manager) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->contextRepository = $context_repository;
+    $this->blockManager = $block_manager;
+    $this->uuidGenerator = $uuid;
+    $this->classResolver = $class_resolver;
+    $this->pluginFormFactory = $plugin_form_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('context.repository'),
+      $container->get('plugin.manager.block'),
+      $container->get('uuid'),
+      $container->get('class_resolver'),
+      $container->get('plugin_form.factory')
+    );
+  }
+
+  /**
+   * Builds the form for the block.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage being configured.
+   * @param int $delta
+   *   The delta of the section.
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The section component containing the block.
+   *
+   * @return array
+   *   The form array.
+   */
+  public function doBuildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL, $delta = NULL, SectionComponent $component = NULL) {
+    $this->sectionStorage = $section_storage;
+    $this->delta = $delta;
+    $this->uuid = $component->getUuid();
+    $this->block = $component->getPlugin();
+
+    $form_state->setTemporaryValue('gathered_contexts', $this->getAvailableContexts($section_storage));
+
+    // @todo Remove once https://www.drupal.org/node/2268787 is resolved.
+    $form_state->set('block_theme', $this->config('system.theme')->get('default'));
+
+    $form['#tree'] = TRUE;
+    $form['settings'] = [];
+    $subform_state = SubformState::createForSubform($form['settings'], $form, $form_state);
+    $form['settings'] = $this->getPluginForm($this->block)->buildConfigurationForm($form['settings'], $subform_state);
+
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->submitLabel(),
+      '#button_type' => 'primary',
+    ];
+    if ($this->isAjax()) {
+      $form['actions']['submit']['#ajax']['callback'] = '::ajaxSubmit';
+    }
+
+    return $form;
+  }
+
+  /**
+   * Returns the label for the submit button.
+   *
+   * @return string
+   *   Submit label.
+   */
+  abstract protected function submitLabel();
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $subform_state = SubformState::createForSubform($form['settings'], $form, $form_state);
+    $this->getPluginForm($this->block)->validateConfigurationForm($form['settings'], $subform_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Call the plugin submit handler.
+    $subform_state = SubformState::createForSubform($form['settings'], $form, $form_state);
+    $this->getPluginForm($this->block)->submitConfigurationForm($form, $subform_state);
+
+    // If this block is context-aware, set the context mapping.
+    if ($this->block instanceof ContextAwarePluginInterface) {
+      $this->block->setContextMapping($subform_state->getValue('context_mapping', []));
+    }
+
+    $configuration = $this->block->getConfiguration();
+
+    $section = $this->sectionStorage->getSection($this->delta);
+    $section->getComponent($this->uuid)->setConfiguration($configuration);
+
+    $this->layoutTempstoreRepository->set($this->sectionStorage);
+    $form_state->setRedirectUrl($this->sectionStorage->getLayoutBuilderUrl());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function successfulAjaxSubmit(array $form, FormStateInterface $form_state) {
+    return $this->rebuildAndClose($this->sectionStorage);
+  }
+
+  /**
+   * Retrieves the plugin form for a given block.
+   *
+   * @param \Drupal\Core\Block\BlockPluginInterface $block
+   *   The block plugin.
+   *
+   * @return \Drupal\Core\Plugin\PluginFormInterface
+   *   The plugin form for the block.
+   */
+  protected function getPluginForm(BlockPluginInterface $block) {
+    if ($block instanceof PluginWithFormsInterface) {
+      return $this->pluginFormFactory->createInstance($block, 'configure');
+    }
+    return $block;
+  }
+
+}

core/modules/layout_builder/src/Form/ConfigureSectionForm.php

@@ -0,0 +1,196 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\DependencyInjection\ClassResolverInterface;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Form\SubformState;
+use Drupal\Core\Layout\LayoutInterface;
+use Drupal\Core\Plugin\PluginFormFactoryInterface;
+use Drupal\Core\Plugin\PluginFormInterface;
+use Drupal\Core\Plugin\PluginWithFormsInterface;
+use Drupal\layout_builder\Controller\LayoutRebuildTrait;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a form for configuring a layout section.
+ *
+ * @internal
+ */
+class ConfigureSectionForm extends FormBase {
+
+  use AjaxFormHelperTrait;
+  use LayoutRebuildTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The plugin being configured.
+   *
+   * @var \Drupal\Core\Layout\LayoutInterface|\Drupal\Core\Plugin\PluginFormInterface
+   */
+  protected $layout;
+
+  /**
+   * The plugin form manager.
+   *
+   * @var \Drupal\Core\Plugin\PluginFormFactoryInterface
+   */
+  protected $pluginFormFactory;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * The field delta.
+   *
+   * @var int
+   */
+  protected $delta;
+
+  /**
+   * Indicates whether the section is being added or updated.
+   *
+   * @var bool
+   */
+  protected $isUpdate;
+
+  /**
+   * Constructs a new ConfigureSectionForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
+   *   The class resolver.
+   * @param \Drupal\Core\Plugin\PluginFormFactoryInterface $plugin_form_manager
+   *   The plugin form manager.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, ClassResolverInterface $class_resolver, PluginFormFactoryInterface $plugin_form_manager) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->classResolver = $class_resolver;
+    $this->pluginFormFactory = $plugin_form_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('class_resolver'),
+      $container->get('plugin_form.factory')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_configure_section';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL, $delta = NULL, $plugin_id = NULL) {
+    $this->sectionStorage = $section_storage;
+    $this->delta = $delta;
+    $this->isUpdate = is_null($plugin_id);
+
+    if ($this->isUpdate) {
+      $section = $this->sectionStorage->getSection($this->delta);
+    }
+    else {
+      $section = new Section($plugin_id);
+    }
+    $this->layout = $section->getLayout();
+
+    $form['#tree'] = TRUE;
+    $form['layout_settings'] = [];
+    $subform_state = SubformState::createForSubform($form['layout_settings'], $form, $form_state);
+    $form['layout_settings'] = $this->getPluginForm($this->layout)->buildConfigurationForm($form['layout_settings'], $subform_state);
+
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#value' => $this->isUpdate ? $this->t('Update') : $this->t('Add section'),
+      '#button_type' => 'primary',
+    ];
+    if ($this->isAjax()) {
+      $form['actions']['submit']['#ajax']['callback'] = '::ajaxSubmit';
+    }
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $subform_state = SubformState::createForSubform($form['layout_settings'], $form, $form_state);
+    $this->getPluginForm($this->layout)->validateConfigurationForm($form['layout_settings'], $subform_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Call the plugin submit handler.
+    $subform_state = SubformState::createForSubform($form['layout_settings'], $form, $form_state);
+    $this->getPluginForm($this->layout)->submitConfigurationForm($form['layout_settings'], $subform_state);
+
+    $plugin_id = $this->layout->getPluginId();
+    $configuration = $this->layout->getConfiguration();
+
+    if ($this->isUpdate) {
+      $this->sectionStorage->getSection($this->delta)->setLayoutSettings($configuration);
+    }
+    else {
+      $this->sectionStorage->insertSection($this->delta, new Section($plugin_id, $configuration));
+    }
+
+    $this->layoutTempstoreRepository->set($this->sectionStorage);
+    $form_state->setRedirectUrl($this->sectionStorage->getLayoutBuilderUrl());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function successfulAjaxSubmit(array $form, FormStateInterface $form_state) {
+    return $this->rebuildAndClose($this->sectionStorage);
+  }
+
+  /**
+   * Retrieves the plugin form for a given layout.
+   *
+   * @param \Drupal\Core\Layout\LayoutInterface $layout
+   *   The layout plugin.
+   *
+   * @return \Drupal\Core\Plugin\PluginFormInterface
+   *   The plugin form for the layout.
+   */
+  protected function getPluginForm(LayoutInterface $layout) {
+    if ($layout instanceof PluginWithFormsInterface) {
+      return $this->pluginFormFactory->createInstance($layout, 'configure');
+    }
+
+    if ($layout instanceof PluginFormInterface) {
+      return $layout;
+    }
+
+    throw new \InvalidArgumentException(sprintf('The "%s" layout does not provide a configuration form', $layout->getPluginId()));
+  }
+
+}

core/modules/layout_builder/src/Form/LayoutBuilderEntityViewDisplayForm.php

@@ -0,0 +1,137 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\field_ui\Form\EntityViewDisplayEditForm;
+use Drupal\layout_builder\Entity\LayoutEntityDisplayInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Edit form for the LayoutBuilderEntityViewDisplay entity type.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class LayoutBuilderEntityViewDisplayForm extends EntityViewDisplayEditForm {
+
+  /**
+   * The entity being used by this form.
+   *
+   * @var \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface
+   */
+  protected $entity;
+
+  /**
+   * The storage section.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL) {
+    $this->sectionStorage = $section_storage;
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function form(array $form, FormStateInterface $form_state) {
+    $form = parent::form($form, $form_state);
+
+    // Hide the table of fields.
+    $form['fields']['#access'] = FALSE;
+    $form['#fields'] = [];
+    $form['#extra'] = [];
+
+    $form['manage_layout'] = [
+      '#type' => 'link',
+      '#title' => $this->t('Manage layout'),
+      '#weight' => -10,
+      '#attributes' => ['class' => ['button']],
+      '#url' => $this->sectionStorage->getLayoutBuilderUrl(),
+    ];
+
+    // @todo Expand to work for all view modes in
+    //   https://www.drupal.org/node/2907413.
+    if ($this->entity->getMode() === 'default') {
+      $form['layout'] = [
+        '#type' => 'details',
+        '#open' => TRUE,
+        '#title' => $this->t('Layout options'),
+        '#tree' => TRUE,
+      ];
+
+      $entity_type = $this->entityTypeManager->getDefinition($this->entity->getTargetEntityTypeId());
+      $form['layout']['allow_custom'] = [
+        '#type' => 'checkbox',
+        '#title' => $this->t('Allow each @entity to have its layout customized.', [
+          '@entity' => $entity_type->getSingularLabel(),
+        ]),
+        '#default_value' => $this->entity->isOverridable(),
+      ];
+      // Prevent turning off overrides while any exist.
+      if ($this->hasOverrides($this->entity)) {
+        $form['layout']['allow_custom']['#disabled'] = TRUE;
+        $form['layout']['allow_custom']['#description'] = $this->t('You must revert all customized layouts of this display before you can disable this option.');
+      }
+      else {
+        $form['#entity_builders'][] = '::entityFormEntityBuild';
+      }
+    }
+    return $form;
+  }
+
+  /**
+   * Determines if the defaults have any overrides.
+   *
+   * @param \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface $display
+   *   The entity display.
+   *
+   * @return bool
+   *   TRUE if there are any overrides of this default, FALSE otherwise.
+   */
+  protected function hasOverrides(LayoutEntityDisplayInterface $display) {
+    if (!$display->isOverridable()) {
+      return FALSE;
+    }
+
+    $entity_type = $this->entityTypeManager->getDefinition($display->getTargetEntityTypeId());
+    $query = $this->entityTypeManager->getStorage($display->getTargetEntityTypeId())->getQuery()
+      ->exists('layout_builder__layout');
+    if ($bundle_key = $entity_type->getKey('bundle')) {
+      $query->condition($bundle_key, $display->getTargetBundle());
+    }
+    return (bool) $query->count()->execute();
+  }
+
+  /**
+   * Entity builder for layout options on the entity view display form.
+   */
+  public function entityFormEntityBuild($entity_type_id, LayoutEntityDisplayInterface $display, &$form, FormStateInterface &$form_state) {
+    $new_value = (bool) $form_state->getValue(['layout', 'allow_custom'], FALSE);
+    $display->setOverridable($new_value);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildFieldRow(FieldDefinitionInterface $field_definition, array $form, FormStateInterface $form_state) {
+    // Intentionally empty.
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildExtraFieldRow($field_id, $extra_field) {
+    // Intentionally empty.
+  }
+
+}

core/modules/layout_builder/src/Form/LayoutRebuildConfirmFormBase.php

@@ -0,0 +1,119 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\DependencyInjection\ClassResolverInterface;
+use Drupal\Core\Form\ConfirmFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\Controller\LayoutRebuildTrait;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a base class for confirmation forms that rebuild the Layout Builder.
+ *
+ * @internal
+ */
+abstract class LayoutRebuildConfirmFormBase extends ConfirmFormBase {
+
+  use AjaxFormHelperTrait;
+  use LayoutRebuildTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * The field delta.
+   *
+   * @var int
+   */
+  protected $delta;
+
+  /**
+   * Constructs a new RemoveSectionForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\DependencyInjection\ClassResolverInterface $class_resolver
+   *   The class resolver.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, ClassResolverInterface $class_resolver) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->classResolver = $class_resolver;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('class_resolver')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCancelUrl() {
+    return $this->sectionStorage->getLayoutBuilderUrl()->setOption('query', ['layout_is_rebuilding' => TRUE]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL, $delta = NULL) {
+    $this->sectionStorage = $section_storage;
+    $this->delta = $delta;
+
+    $form = parent::buildForm($form, $form_state);
+
+    if ($this->isAjax()) {
+      $form['actions']['submit']['#ajax']['callback'] = '::ajaxSubmit';
+      $form['actions']['cancel']['#attributes']['class'][] = 'dialog-cancel';
+    }
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->handleSectionStorage($this->sectionStorage, $form_state);
+
+    $this->layoutTempstoreRepository->set($this->sectionStorage);
+
+    $form_state->setRedirectUrl($this->getCancelUrl());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function successfulAjaxSubmit(array $form, FormStateInterface $form_state) {
+    return $this->rebuildAndClose($this->sectionStorage);
+  }
+
+  /**
+   * Performs any actions on the section storage before saving.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   */
+  abstract protected function handleSectionStorage(SectionStorageInterface $section_storage, FormStateInterface $form_state);
+
+}

core/modules/layout_builder/src/Form/RemoveBlockForm.php

@@ -0,0 +1,66 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a form to confirm the removal of a block.
+ *
+ * @internal
+ */
+class RemoveBlockForm extends LayoutRebuildConfirmFormBase {
+
+  /**
+   * The current region.
+   *
+   * @var string
+   */
+  protected $region;
+
+  /**
+   * The UUID of the block being removed.
+   *
+   * @var string
+   */
+  protected $uuid;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    return $this->t('Are you sure you want to remove this block?');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfirmText() {
+    return $this->t('Remove');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_remove_block';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL, $delta = NULL, $region = NULL, $uuid = NULL) {
+    $this->region = $region;
+    $this->uuid = $uuid;
+    return parent::buildForm($form, $form_state, $section_storage, $delta);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function handleSectionStorage(SectionStorageInterface $section_storage, FormStateInterface $form_state) {
+    $section_storage->getSection($this->delta)->removeComponent($this->uuid);
+  }
+
+}

core/modules/layout_builder/src/Form/RemoveSectionForm.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a form to confirm the removal of a section.
+ *
+ * @internal
+ */
+class RemoveSectionForm extends LayoutRebuildConfirmFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_remove_section';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    return $this->t('Are you sure you want to remove this section?');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfirmText() {
+    return $this->t('Remove');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function handleSectionStorage(SectionStorageInterface $section_storage, FormStateInterface $form_state) {
+    $section_storage->removeSection($this->delta);
+  }
+
+}

core/modules/layout_builder/src/Form/RevertOverridesForm.php

@@ -0,0 +1,117 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\ConfirmFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Reverts the overridden layout to the defaults.
+ */
+class RevertOverridesForm extends ConfirmFormBase {
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The messenger service.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * The section storage.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * Constructs a new RevertOverridesForm.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->messenger = $messenger;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('layout_builder.tempstore_repository'),
+      $container->get('messenger')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_revert_overrides';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    return $this->t('Are you sure you want to revert this to defaults?');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfirmText() {
+    return $this->t('Revert');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCancelUrl() {
+    return $this->sectionStorage->getLayoutBuilderUrl();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL) {
+    if (!$section_storage instanceof OverridesSectionStorageInterface) {
+      throw new \InvalidArgumentException(sprintf('The section storage with type "%s" and ID "%s" does not provide overrides', $section_storage->getStorageType(), $section_storage->getStorageId()));
+    }
+
+    $this->sectionStorage = $section_storage;
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Remove all sections.
+    while ($this->sectionStorage->count()) {
+      $this->sectionStorage->removeSection(0);
+    }
+    $this->sectionStorage->save();
+    $this->layoutTempstoreRepository->delete($this->sectionStorage);
+
+    $this->messenger->addMessage($this->t('The layout has been reverted back to defaults.'));
+    $form_state->setRedirectUrl($this->getCancelUrl());
+  }
+
+}

core/modules/layout_builder/src/Form/UpdateBlockForm.php

@@ -0,0 +1,53 @@
+<?php
+
+namespace Drupal\layout_builder\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a form to update a block.
+ *
+ * @internal
+ */
+class UpdateBlockForm extends ConfigureBlockFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'layout_builder_update_block';
+  }
+
+  /**
+   * Builds the block form.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage being configured.
+   * @param int $delta
+   *   The delta of the section.
+   * @param string $region
+   *   The region of the block.
+   * @param string $uuid
+   *   The UUID of the block being updated.
+   *
+   * @return array
+   *   The form array.
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, SectionStorageInterface $section_storage = NULL, $delta = NULL, $region = NULL, $uuid = NULL) {
+    $component = $section_storage->getSection($delta)->getComponent($uuid);
+    return $this->doBuildForm($form, $form_state, $section_storage, $delta, $component);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function submitLabel() {
+    return $this->t('Update');
+  }
+
+}

core/modules/layout_builder/src/LayoutBuilderEvents.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Defines events for the layout_builder module.
+ *
+ * @see \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+final class LayoutBuilderEvents {
+
+  /**
+   * Name of the event fired when a component's render array is built.
+   *
+   * This event allows modules to collaborate on creating the render array of
+   * the SectionComponent object. The event listener method receives a
+   * \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent
+   * instance.
+   *
+   * @Event
+   *
+   * @see \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent
+   * @see \Drupal\layout_builder\SectionComponent::toRenderArray()
+   *
+   * @var string
+   */
+  const SECTION_COMPONENT_BUILD_RENDER_ARRAY = 'section_component.build.render_array';
+
+}

core/modules/layout_builder/src/LayoutTempstoreRepository.php

@@ -0,0 +1,78 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Core\TempStore\SharedTempStoreFactory;
+
+/**
+ * Provides a mechanism for loading layouts from tempstore.
+ *
+ * @internal
+ */
+class LayoutTempstoreRepository implements LayoutTempstoreRepositoryInterface {
+
+  /**
+   * The shared tempstore factory.
+   *
+   * @var \Drupal\Core\TempStore\SharedTempStoreFactory
+   */
+  protected $tempStoreFactory;
+
+  /**
+   * LayoutTempstoreRepository constructor.
+   *
+   * @param \Drupal\Core\TempStore\SharedTempStoreFactory $temp_store_factory
+   *   The shared tempstore factory.
+   */
+  public function __construct(SharedTempStoreFactory $temp_store_factory) {
+    $this->tempStoreFactory = $temp_store_factory;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function get(SectionStorageInterface $section_storage) {
+    $id = $section_storage->getStorageId();
+    $tempstore = $this->getTempstore($section_storage)->get($id);
+    if (!empty($tempstore['section_storage'])) {
+      $storage_type = $section_storage->getStorageType();
+      $section_storage = $tempstore['section_storage'];
+
+      if (!($section_storage instanceof SectionStorageInterface)) {
+        throw new \UnexpectedValueException(sprintf('The entry with storage type "%s" and ID "%s" is invalid', $storage_type, $id));
+      }
+    }
+    return $section_storage;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function set(SectionStorageInterface $section_storage) {
+    $id = $section_storage->getStorageId();
+    $this->getTempstore($section_storage)->set($id, ['section_storage' => $section_storage]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function delete(SectionStorageInterface $section_storage) {
+    $id = $section_storage->getStorageId();
+    $this->getTempstore($section_storage)->delete($id);
+  }
+
+  /**
+   * Gets the shared tempstore.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage.
+   *
+   * @return \Drupal\Core\TempStore\SharedTempStore
+   *   The tempstore.
+   */
+  protected function getTempstore(SectionStorageInterface $section_storage) {
+    $collection = 'layout_builder.section_storage.' . $section_storage->getStorageType();
+    return $this->tempStoreFactory->get($collection);
+  }
+
+}

core/modules/layout_builder/src/LayoutTempstoreRepositoryInterface.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Provides an interface for loading layouts from tempstore.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+interface LayoutTempstoreRepositoryInterface {
+
+  /**
+   * Gets the tempstore version of a section storage, if it exists.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage to check for in tempstore.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   Either the version of this section storage from tempstore, or the passed
+   *   section storage if none exists.
+   *
+   * @throw \UnexpectedValueException
+   *   Thrown if a value exists, but is not a section storage.
+   */
+  public function get(SectionStorageInterface $section_storage);
+
+  /**
+   * Stores this section storage in tempstore.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage to set in tempstore.
+   */
+  public function set(SectionStorageInterface $section_storage);
+
+  /**
+   * Removes the tempstore version of a section storage.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage to remove from tempstore.
+   */
+  public function delete(SectionStorageInterface $section_storage);
+
+}

core/modules/layout_builder/src/OverridesSectionStorageInterface.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Defines an interface for an object that stores layout sections for overrides.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+interface OverridesSectionStorageInterface extends SectionStorageInterface {
+
+  /**
+   * Returns the corresponding defaults section storage for this override.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The defaults section storage.
+   *
+   * @todo Determine if this method needs a parameter in
+   *   https://www.drupal.org/project/drupal/issues/2936507.
+   */
+  public function getDefaultSectionStorage();
+
+}

core/modules/layout_builder/src/Plugin/Block/FieldBlock.php

@@ -0,0 +1,377 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Block;
+
+use Drupal\Component\Plugin\Factory\DefaultFactory;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\Entity\EntityDisplayBase;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FormatterInterface;
+use Drupal\Core\Field\FormatterPluginManager;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Render\Element;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides a block that renders a field from an entity.
+ *
+ * @Block(
+ *   id = "field_block",
+ *   deriver = "\Drupal\layout_builder\Plugin\Derivative\FieldBlockDeriver",
+ * )
+ */
+class FieldBlock extends BlockBase implements ContextAwarePluginInterface, ContainerFactoryPluginInterface {
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The formatter manager.
+   *
+   * @var \Drupal\Core\Field\FormatterPluginManager
+   */
+  protected $formatterManager;
+
+  /**
+   * The entity type ID.
+   *
+   * @var string
+   */
+  protected $entityTypeId;
+
+  /**
+   * The bundle ID.
+   *
+   * @var string
+   */
+  protected $bundle;
+
+  /**
+   * The field name.
+   *
+   * @var string
+   */
+  protected $fieldName;
+
+  /**
+   * The field definition.
+   *
+   * @var \Drupal\Core\Field\FieldDefinitionInterface
+   */
+  protected $fieldDefinition;
+
+  /**
+   * The module handler.
+   *
+   * @var \Drupal\Core\Extension\ModuleHandlerInterface
+   */
+  protected $moduleHandler;
+
+  /**
+   * Constructs a new FieldBlock.
+   *
+   * @param array $configuration
+   *   A configuration array containing information about the plugin instance.
+   * @param string $plugin_id
+   *   The plugin ID for the plugin instance.
+   * @param mixed $plugin_definition
+   *   The plugin implementation definition.
+   * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
+   *   The entity field manager.
+   * @param \Drupal\Core\Field\FormatterPluginManager $formatter_manager
+   *   The formatter manager.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityFieldManagerInterface $entity_field_manager, FormatterPluginManager $formatter_manager, ModuleHandlerInterface $module_handler) {
+    $this->entityFieldManager = $entity_field_manager;
+    $this->formatterManager = $formatter_manager;
+    $this->moduleHandler = $module_handler;
+
+    // Get the entity type and field name from the plugin ID.
+    list (, $entity_type_id, $bundle, $field_name) = explode(static::DERIVATIVE_SEPARATOR, $plugin_id, 4);
+    $this->entityTypeId = $entity_type_id;
+    $this->bundle = $bundle;
+    $this->fieldName = $field_name;
+
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('entity_field.manager'),
+      $container->get('plugin.manager.field.formatter'),
+      $container->get('module_handler')
+    );
+  }
+
+  /**
+   * Gets the entity that has the field.
+   *
+   * @return \Drupal\Core\Entity\FieldableEntityInterface
+   *   The entity.
+   */
+  protected function getEntity() {
+    return $this->getContextValue('entity');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $display_settings = $this->getConfiguration()['formatter'];
+    $entity = $this->getEntity();
+    $build = $entity->get($this->fieldName)->view($display_settings);
+    if (!empty($entity->in_preview) && !Element::getVisibleChildren($build)) {
+      $build['content']['#markup'] = new TranslatableMarkup('Placeholder for the "@field" field', ['@field' => $this->getFieldDefinition()->getLabel()]);
+    }
+    CacheableMetadata::createFromObject($this)->applyTo($build);
+    return $build;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function blockAccess(AccountInterface $account) {
+    $entity = $this->getEntity();
+
+    // First consult the entity.
+    $access = $entity->access('view', $account, TRUE);
+    if (!$access->isAllowed()) {
+      return $access;
+    }
+
+    // Check that the entity in question has this field.
+    if (!$entity instanceof FieldableEntityInterface || !$entity->hasField($this->fieldName)) {
+      return $access->andIf(AccessResult::forbidden());
+    }
+
+    // Check field access.
+    $field = $entity->get($this->fieldName);
+    $access = $access->andIf($field->access('view', $account, TRUE));
+    if (!$access->isAllowed()) {
+      return $access;
+    }
+
+    // Check to see if the field has any values.
+    if ($field->isEmpty()) {
+      return $access->andIf(AccessResult::forbidden());
+    }
+    return $access;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'label_display' => FALSE,
+      'formatter' => [
+        'label' => 'above',
+        'type' => $this->pluginDefinition['default_formatter'],
+        'settings' => [],
+        'third_party_settings' => [],
+      ],
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockForm($form, FormStateInterface $form_state) {
+    $config = $this->getConfiguration();
+
+    $form['formatter'] = [
+      '#tree' => TRUE,
+      '#process' => [
+        [$this, 'formatterSettingsProcessCallback'],
+      ],
+    ];
+    $form['formatter']['label'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Label'),
+      // @todo This is directly copied from
+      //   \Drupal\field_ui\Form\EntityViewDisplayEditForm::getFieldLabelOptions(),
+      //   resolve this in https://www.drupal.org/project/drupal/issues/2933924.
+      '#options' => [
+        'above' => $this->t('Above'),
+        'inline' => $this->t('Inline'),
+        'hidden' => '- ' . $this->t('Hidden') . ' -',
+        'visually_hidden' => '- ' . $this->t('Visually Hidden') . ' -',
+      ],
+      '#default_value' => $config['formatter']['label'],
+    ];
+
+    $form['formatter']['type'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Formatter'),
+      '#options' => $this->getApplicablePluginOptions($this->getFieldDefinition()),
+      '#required' => TRUE,
+      '#default_value' => $config['formatter']['type'],
+      '#ajax' => [
+        'callback' => [static::class, 'formatterSettingsAjaxCallback'],
+        'wrapper' => 'formatter-settings-wrapper',
+      ],
+    ];
+
+    // Add the formatter settings to the form via AJAX.
+    $form['formatter']['settings_wrapper'] = [
+      '#prefix' => '<div id="formatter-settings-wrapper">',
+      '#suffix' => '</div>',
+    ];
+
+    return $form;
+  }
+
+  /**
+   * Render API callback: builds the formatter settings elements.
+   */
+  public function formatterSettingsProcessCallback(array &$element, FormStateInterface $form_state, array &$complete_form) {
+    if ($formatter = $this->getFormatter($element['#parents'], $form_state)) {
+      $element['settings_wrapper']['settings'] = $formatter->settingsForm($complete_form, $form_state);
+      $element['settings_wrapper']['settings']['#parents'] = array_merge($element['#parents'], ['settings']);
+      $element['settings_wrapper']['third_party_settings'] = $this->thirdPartySettingsForm($formatter, $this->getFieldDefinition(), $complete_form, $form_state);
+      $element['settings_wrapper']['third_party_settings']['#parents'] = array_merge($element['#parents'], ['third_party_settings']);
+
+      // Store the array parents for our element so that we can retrieve the
+      // formatter settings in our AJAX callback.
+      $form_state->set('field_block_array_parents', $element['#array_parents']);
+    }
+    return $element;
+  }
+
+  /**
+   * Adds the formatter third party settings forms.
+   *
+   * @param \Drupal\Core\Field\FormatterInterface $plugin
+   *   The formatter.
+   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
+   *   The field definition.
+   * @param array $form
+   *   The (entire) configuration form array.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   *
+   * @return array
+   *   The formatter third party settings form.
+   */
+  protected function thirdPartySettingsForm(FormatterInterface $plugin, FieldDefinitionInterface $field_definition, array $form, FormStateInterface $form_state) {
+    $settings_form = [];
+    // Invoke hook_field_formatter_third_party_settings_form(), keying resulting
+    // subforms by module name.
+    foreach ($this->moduleHandler->getImplementations('field_formatter_third_party_settings_form') as $module) {
+      $settings_form[$module] = $this->moduleHandler->invoke($module, 'field_formatter_third_party_settings_form', [
+        $plugin,
+        $field_definition,
+        EntityDisplayBase::CUSTOM_MODE,
+        $form,
+        $form_state,
+      ]);
+    }
+    return $settings_form;
+  }
+
+  /**
+   * Render API callback: gets the layout settings elements.
+   */
+  public static function formatterSettingsAjaxCallback(array $form, FormStateInterface $form_state) {
+    $formatter_array_parents = $form_state->get('field_block_array_parents');
+    return NestedArray::getValue($form, array_merge($formatter_array_parents, ['settings_wrapper']));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockSubmit($form, FormStateInterface $form_state) {
+    $this->configuration['formatter'] = $form_state->getValue('formatter');
+  }
+
+  /**
+   * Gets the field definition.
+   *
+   * @return \Drupal\Core\Field\FieldDefinitionInterface
+   *   The field definition.
+   */
+  protected function getFieldDefinition() {
+    if (empty($this->fieldDefinition)) {
+      $field_definitions = $this->entityFieldManager->getFieldDefinitions($this->entityTypeId, $this->bundle);
+      $this->fieldDefinition = $field_definitions[$this->fieldName];
+    }
+    return $this->fieldDefinition;
+  }
+
+  /**
+   * Returns an array of applicable formatter options for a field.
+   *
+   * @param \Drupal\Core\Field\FieldDefinitionInterface $field_definition
+   *   The field definition.
+   *
+   * @return array
+   *   An array of applicable formatter options.
+   *
+   * @see \Drupal\field_ui\Form\EntityDisplayFormBase::getApplicablePluginOptions()
+   */
+  protected function getApplicablePluginOptions(FieldDefinitionInterface $field_definition) {
+    $options = $this->formatterManager->getOptions($field_definition->getType());
+    $applicable_options = [];
+    foreach ($options as $option => $label) {
+      $plugin_class = DefaultFactory::getPluginClass($option, $this->formatterManager->getDefinition($option));
+      if ($plugin_class::isApplicable($field_definition)) {
+        $applicable_options[$option] = $label;
+      }
+    }
+    return $applicable_options;
+  }
+
+  /**
+   * Gets the formatter object.
+   *
+   * @param array $parents
+   *   The #parents of the element representing the formatter.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   *
+   * @return \Drupal\Core\Field\FormatterInterface
+   *   The formatter object.
+   */
+  protected function getFormatter(array $parents, FormStateInterface $form_state) {
+    // Use the processed values, if available.
+    $configuration = NestedArray::getValue($form_state->getValues(), $parents);
+    if (!$configuration) {
+      // Next check the raw user input.
+      $configuration = NestedArray::getValue($form_state->getUserInput(), $parents);
+      if (!$configuration) {
+        // If no user input exists, use the default values.
+        $configuration = $this->getConfiguration()['formatter'];
+      }
+    }
+
+    return $this->formatterManager->getInstance([
+      'configuration' => $configuration,
+      'field_definition' => $this->getFieldDefinition(),
+      'view_mode' => EntityDisplayBase::CUSTOM_MODE,
+      'prepare' => TRUE,
+    ]);
+  }
+
+}

core/modules/layout_builder/src/Plugin/DataType/SectionData.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\DataType;
+
+use Drupal\Core\TypedData\TypedData;
+use Drupal\layout_builder\Section;
+
+/**
+ * Provides a data type wrapping \Drupal\layout_builder\Section.
+ *
+ * @DataType(
+ *   id = "layout_section",
+ *   label = @Translation("Layout Section"),
+ *   description = @Translation("A layout section"),
+ * )
+ */
+class SectionData extends TypedData {
+
+  /**
+   * The section object.
+   *
+   * @var \Drupal\layout_builder\Section
+   */
+  protected $value;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setValue($value, $notify = TRUE) {
+    if ($value && !$value instanceof Section) {
+      throw new \InvalidArgumentException(sprintf('Value assigned to "%s" is not a valid section', $this->getName()));
+    }
+    parent::setValue($value, $notify);
+  }
+
+}

core/modules/layout_builder/src/Plugin/Derivative/FieldBlockDeriver.php

@@ -0,0 +1,137 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Derivative;
+
+use Drupal\Component\Plugin\Derivative\DeriverBase;
+use Drupal\Component\Plugin\PluginBase;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeRepositoryInterface;
+use Drupal\Core\Field\FieldConfigInterface;
+use Drupal\Core\Field\FieldTypePluginManagerInterface;
+use Drupal\Core\Field\FormatterPluginManager;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides entity field block definitions for every field.
+ *
+ * @internal
+ */
+class FieldBlockDeriver extends DeriverBase implements ContainerDeriverInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The entity type repository.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeRepositoryInterface
+   */
+  protected $entityTypeRepository;
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * The field type manager.
+   *
+   * @var \Drupal\Core\Field\FieldTypePluginManagerInterface
+   */
+  protected $fieldTypeManager;
+
+  /**
+   * The formatter manager.
+   *
+   * @var \Drupal\Core\Field\FormatterPluginManager
+   */
+  protected $formatterManager;
+
+  /**
+   * Constructs new FieldBlockDeriver.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeRepositoryInterface $entity_type_repository
+   *   The entity type repository.
+   * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
+   *   The entity field manager.
+   * @param \Drupal\Core\Field\FieldTypePluginManagerInterface $field_type_manager
+   *   The field type manager.
+   * @param \Drupal\Core\Field\FormatterPluginManager $formatter_manager
+   *   The formatter manager.
+   */
+  public function __construct(EntityTypeRepositoryInterface $entity_type_repository, EntityFieldManagerInterface $entity_field_manager, FieldTypePluginManagerInterface $field_type_manager, FormatterPluginManager $formatter_manager) {
+    $this->entityTypeRepository = $entity_type_repository;
+    $this->entityFieldManager = $entity_field_manager;
+    $this->fieldTypeManager = $field_type_manager;
+    $this->formatterManager = $formatter_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, $base_plugin_id) {
+    return new static(
+      $container->get('entity_type.repository'),
+      $container->get('entity_field.manager'),
+      $container->get('plugin.manager.field.field_type'),
+      $container->get('plugin.manager.field.formatter')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDerivativeDefinitions($base_plugin_definition) {
+    $entity_type_labels = $this->entityTypeRepository->getEntityTypeLabels();
+    foreach ($this->entityFieldManager->getFieldMap() as $entity_type_id => $entity_field_map) {
+      foreach ($entity_field_map as $field_name => $field_info) {
+        // Skip fields without any formatters.
+        $options = $this->formatterManager->getOptions($field_info['type']);
+        if (empty($options)) {
+          continue;
+        }
+
+        foreach ($field_info['bundles'] as $bundle) {
+          $derivative = $base_plugin_definition;
+          $field_definition = $this->entityFieldManager->getFieldDefinitions($entity_type_id, $bundle)[$field_name];
+
+          // Store the default formatter on the definition.
+          $derivative['default_formatter'] = '';
+          $field_type_definition = $this->fieldTypeManager->getDefinition($field_info['type']);
+          if (isset($field_type_definition['default_formatter'])) {
+            $derivative['default_formatter'] = $field_type_definition['default_formatter'];
+          }
+
+          $derivative['category'] = $this->t('@entity', ['@entity' => $entity_type_labels[$entity_type_id]]);
+
+          $derivative['admin_label'] = $field_definition->getLabel();
+
+          // Add a dependency on the field if it is configurable.
+          if ($field_definition instanceof FieldConfigInterface) {
+            $derivative['config_dependencies'][$field_definition->getConfigDependencyKey()][] = $field_definition->getConfigDependencyName();
+          }
+          // For any field that is not display configurable, mark it as
+          // unavailable to place in the block UI.
+          $derivative['_block_ui_hidden'] = !$field_definition->isDisplayConfigurable('view');
+
+          // @todo Use EntityContextDefinition after resolving
+          //   https://www.drupal.org/node/2932462.
+          $context_definition = new ContextDefinition('entity:' . $entity_type_id, $entity_type_labels[$entity_type_id], TRUE);
+          $context_definition->addConstraint('Bundle', [$bundle]);
+          $derivative['context'] = [
+            'entity' => $context_definition,
+          ];
+
+          $derivative_id = $entity_type_id . PluginBase::DERIVATIVE_SEPARATOR . $bundle . PluginBase::DERIVATIVE_SEPARATOR . $field_name;
+          $this->derivatives[$derivative_id] = $derivative;
+        }
+      }
+    }
+    return $this->derivatives;
+  }
+
+}

core/modules/layout_builder/src/Plugin/Derivative/LayoutBuilderLocalTaskDeriver.php

@@ -0,0 +1,134 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Derivative;
+
+use Drupal\Component\Plugin\Derivative\DeriverBase;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Plugin\Discovery\ContainerDeriverInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Provides local task definitions for the layout builder user interface.
+ *
+ * @todo Remove this in https://www.drupal.org/project/drupal/issues/2936655.
+ *
+ * @internal
+ */
+class LayoutBuilderLocalTaskDeriver extends DeriverBase implements ContainerDeriverInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * Constructs a new LayoutBuilderLocalTaskDeriver.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager) {
+    $this->entityTypeManager = $entity_type_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, $base_plugin_id) {
+    return new static(
+      $container->get('entity_type.manager')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDerivativeDefinitions($base_plugin_definition) {
+    foreach ($this->getEntityTypesForOverrides() as $entity_type_id => $entity_type) {
+      // Overrides.
+      $this->derivatives["layout_builder.overrides.$entity_type_id.view"] = $base_plugin_definition + [
+        'route_name' => "layout_builder.overrides.$entity_type_id.view",
+        'weight' => 15,
+        'title' => $this->t('Layout'),
+        'base_route' => "entity.$entity_type_id.canonical",
+        'cache_contexts' => ['layout_builder_is_active:' . $entity_type_id],
+      ];
+      $this->derivatives["layout_builder.overrides.$entity_type_id.save"] = $base_plugin_definition + [
+        'route_name' => "layout_builder.overrides.$entity_type_id.save",
+        'title' => $this->t('Save Layout'),
+        'parent_id' => "layout_builder_ui:layout_builder.overrides.$entity_type_id.view",
+        'cache_contexts' => ['layout_builder_is_active:' . $entity_type_id],
+      ];
+      $this->derivatives["layout_builder.overrides.$entity_type_id.cancel"] = $base_plugin_definition + [
+        'route_name' => "layout_builder.overrides.$entity_type_id.cancel",
+        'title' => $this->t('Cancel Layout'),
+        'parent_id' => "layout_builder_ui:layout_builder.overrides.$entity_type_id.view",
+        'weight' => 5,
+        'cache_contexts' => ['layout_builder_is_active:' . $entity_type_id],
+      ];
+      // @todo This link should be conditionally displayed, see
+      //   https://www.drupal.org/node/2917777.
+      $this->derivatives["layout_builder.overrides.$entity_type_id.revert"] = $base_plugin_definition + [
+        'route_name' => "layout_builder.overrides.$entity_type_id.revert",
+        'title' => $this->t('Revert to defaults'),
+        'parent_id' => "layout_builder_ui:layout_builder.overrides.$entity_type_id.view",
+        'weight' => 10,
+        'cache_contexts' => ['layout_builder_is_active:' . $entity_type_id],
+      ];
+    }
+
+    foreach ($this->getEntityTypesForDefaults() as $entity_type_id => $entity_type) {
+      // Defaults.
+      $this->derivatives["layout_builder.defaults.$entity_type_id.view"] = $base_plugin_definition + [
+        'route_name' => "layout_builder.defaults.$entity_type_id.view",
+        'title' => $this->t('Manage layout'),
+        'base_route' => "layout_builder.defaults.$entity_type_id.view",
+      ];
+      $this->derivatives["layout_builder.defaults.$entity_type_id.save"] = $base_plugin_definition + [
+        'route_name' => "layout_builder.defaults.$entity_type_id.save",
+        'title' => $this->t('Save Layout'),
+        'parent_id' => "layout_builder_ui:layout_builder.defaults.$entity_type_id.view",
+      ];
+      $this->derivatives["layout_builder.defaults.$entity_type_id.cancel"] = $base_plugin_definition + [
+        'route_name' => "layout_builder.defaults.$entity_type_id.cancel",
+        'title' => $this->t('Cancel Layout'),
+        'weight' => 5,
+        'parent_id' => "layout_builder_ui:layout_builder.defaults.$entity_type_id.view",
+      ];
+    }
+
+    return $this->derivatives;
+  }
+
+  /**
+   * Returns an array of entity types relevant for defaults.
+   *
+   * @return \Drupal\Core\Entity\EntityTypeInterface[]
+   *   An array of entity types.
+   */
+  protected function getEntityTypesForDefaults() {
+    return array_filter($this->entityTypeManager->getDefinitions(), function (EntityTypeInterface $entity_type) {
+      return $entity_type->entityClassImplements(FieldableEntityInterface::class) && $entity_type->hasViewBuilderClass() && $entity_type->get('field_ui_base_route');
+    });
+  }
+
+  /**
+   * Returns an array of entity types relevant for overrides.
+   *
+   * @return \Drupal\Core\Entity\EntityTypeInterface[]
+   *   An array of entity types.
+   */
+  protected function getEntityTypesForOverrides() {
+    return array_filter($this->entityTypeManager->getDefinitions(), function (EntityTypeInterface $entity_type) {
+      return $entity_type->entityClassImplements(FieldableEntityInterface::class) && $entity_type->hasViewBuilderClass() && $entity_type->hasLinkTemplate('canonical');
+    });
+  }
+
+}

core/modules/layout_builder/src/Plugin/Field/FieldType/LayoutSectionItem.php

@@ -0,0 +1,94 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\Field\FieldType;
+
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemBase;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\TypedData\DataDefinition;
+use Drupal\layout_builder\Section;
+
+/**
+ * Plugin implementation of the 'layout_section' field type.
+ *
+ * @internal
+ *
+ * @FieldType(
+ *   id = "layout_section",
+ *   label = @Translation("Layout Section"),
+ *   description = @Translation("Layout Section"),
+ *   list_class = "\Drupal\layout_builder\Field\LayoutSectionItemList",
+ *   no_ui = TRUE,
+ *   cardinality = \Drupal\Core\Field\FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED
+ * )
+ *
+ * @property \Drupal\layout_builder\Section section
+ */
+class LayoutSectionItem extends FieldItemBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
+    $properties['section'] = DataDefinition::create('layout_section')
+      ->setLabel(new TranslatableMarkup('Layout Section'))
+      ->setRequired(FALSE);
+
+    return $properties;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __get($name) {
+    // @todo \Drupal\Core\Field\FieldItemBase::__get() does not return default
+    //   values for uninstantiated properties. This will forcibly instantiate
+    //   all properties with the side-effect of a performance hit, resolve
+    //   properly in https://www.drupal.org/node/2413471.
+    $this->getProperties();
+
+    return parent::__get($name);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function mainPropertyName() {
+    return 'section';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function schema(FieldStorageDefinitionInterface $field_definition) {
+    $schema = [
+      'columns' => [
+        'section' => [
+          'type' => 'blob',
+          'size' => 'normal',
+          'serialize' => TRUE,
+        ],
+      ],
+    ];
+
+    return $schema;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
+    // @todo Expand this in https://www.drupal.org/node/2912331.
+    $values['section'] = new Section('layout_onecol');
+    return $values;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isEmpty() {
+    return empty($this->section);
+  }
+
+}

core/modules/layout_builder/src/Plugin/SectionStorage/DefaultsSectionStorage.php

@@ -0,0 +1,296 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\SectionStorage;
+
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\Context\Context;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Drupal\field_ui\FieldUI;
+use Drupal\layout_builder\DefaultsSectionStorageInterface;
+use Drupal\layout_builder\Entity\LayoutBuilderSampleEntityGenerator;
+use Drupal\layout_builder\Entity\LayoutEntityDisplayInterface;
+use Drupal\layout_builder\SectionListInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Defines the 'defaults' section storage type.
+ *
+ * @SectionStorage(
+ *   id = "defaults",
+ * )
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class DefaultsSectionStorage extends SectionStorageBase implements ContainerFactoryPluginInterface, DefaultsSectionStorageInterface {
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The entity type bundle info.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
+   */
+  protected $entityTypeBundleInfo;
+
+  /**
+   * {@inheritdoc}
+   *
+   * @var \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface
+   */
+  protected $sectionList;
+
+  /**
+   * The sample entity generator.
+   *
+   * @var \Drupal\layout_builder\Entity\LayoutBuilderSampleEntityGenerator
+   */
+  protected $sampleEntityGenerator;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityTypeBundleInfoInterface $entity_type_bundle_info, LayoutBuilderSampleEntityGenerator $sample_entity_generator) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+
+    $this->entityTypeManager = $entity_type_manager;
+    $this->entityTypeBundleInfo = $entity_type_bundle_info;
+    $this->sampleEntityGenerator = $sample_entity_generator;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('entity_type.manager'),
+      $container->get('entity_type.bundle.info'),
+      $container->get('layout_builder.sample_entity_generator')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setSectionList(SectionListInterface $section_list) {
+    if (!$section_list instanceof LayoutEntityDisplayInterface) {
+      throw new \InvalidArgumentException('Defaults expect a display-based section list');
+    }
+
+    return parent::setSectionList($section_list);
+  }
+
+  /**
+   * Gets the entity storing the overrides.
+   *
+   * @return \Drupal\layout_builder\Entity\LayoutEntityDisplayInterface
+   *   The entity storing the defaults.
+   */
+  protected function getDisplay() {
+    return $this->getSectionList();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStorageId() {
+    return $this->getDisplay()->id();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRedirectUrl() {
+    return Url::fromRoute("entity.entity_view_display.{$this->getDisplay()->getTargetEntityTypeId()}.view_mode", $this->getRouteParameters());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLayoutBuilderUrl() {
+    return Url::fromRoute("layout_builder.{$this->getStorageType()}.{$this->getDisplay()->getTargetEntityTypeId()}.view", $this->getRouteParameters());
+  }
+
+  /**
+   * Provides the route parameters needed to generate a URL for this object.
+   *
+   * @return mixed[]
+   *   An associative array of parameter names and values.
+   */
+  protected function getRouteParameters() {
+    $display = $this->getDisplay();
+    $entity_type = $this->entityTypeManager->getDefinition($display->getTargetEntityTypeId());
+    $route_parameters = FieldUI::getRouteBundleParameter($entity_type, $display->getTargetBundle());
+    $route_parameters['view_mode_name'] = $display->getMode();
+    return $route_parameters;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildRoutes(RouteCollection $collection) {
+    foreach ($this->getEntityTypes() as $entity_type_id => $entity_type) {
+      // Try to get the route from the current collection.
+      if (!$entity_route = $collection->get($entity_type->get('field_ui_base_route'))) {
+        continue;
+      }
+
+      $path = $entity_route->getPath() . '/display-layout/{view_mode_name}';
+
+      $defaults = [];
+      $defaults['entity_type_id'] = $entity_type_id;
+      // If the entity type has no bundles and it doesn't use {bundle} in its
+      // admin path, use the entity type.
+      if (strpos($path, '{bundle}') === FALSE) {
+        if (!$entity_type->hasKey('bundle')) {
+          $defaults['bundle'] = $entity_type_id;
+        }
+        else {
+          $defaults['bundle_key'] = $entity_type->getBundleEntityType();
+        }
+      }
+
+      $requirements = [];
+      $requirements['_field_ui_view_mode_access'] = 'administer ' . $entity_type_id . ' display';
+
+      $options = $entity_route->getOptions();
+      $options['_admin_route'] = FALSE;
+
+      $this->buildLayoutRoutes($collection, $this->getPluginDefinition(), $path, $defaults, $requirements, $options, $entity_type_id);
+
+      $route_names = [
+        "entity.entity_view_display.{$entity_type_id}.default",
+        "entity.entity_view_display.{$entity_type_id}.view_mode",
+      ];
+      foreach ($route_names as $route_name) {
+        if (!$route = $collection->get($route_name)) {
+          continue;
+        }
+
+        $route->addDefaults([
+          'section_storage_type' => $this->getStorageType(),
+          'section_storage' => '',
+        ] + $defaults);
+        $parameters['section_storage']['layout_builder_tempstore'] = TRUE;
+        $parameters = NestedArray::mergeDeep($parameters, $route->getOption('parameters') ?: []);
+        $route->setOption('parameters', $parameters);
+      }
+    }
+  }
+
+  /**
+   * Returns an array of relevant entity types.
+   *
+   * @return \Drupal\Core\Entity\EntityTypeInterface[]
+   *   An array of entity types.
+   */
+  protected function getEntityTypes() {
+    return array_filter($this->entityTypeManager->getDefinitions(), function (EntityTypeInterface $entity_type) {
+      return $entity_type->entityClassImplements(FieldableEntityInterface::class) && $entity_type->hasViewBuilderClass() && $entity_type->get('field_ui_base_route');
+    });
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function extractIdFromRoute($value, $definition, $name, array $defaults) {
+    if (is_string($value) && strpos($value, '.') !== FALSE) {
+      return $value;
+    }
+
+    // If a bundle is not provided but a value corresponding to the bundle key
+    // is, use that for the bundle value.
+    if (empty($defaults['bundle']) && isset($defaults['bundle_key']) && !empty($defaults[$defaults['bundle_key']])) {
+      $defaults['bundle'] = $defaults[$defaults['bundle_key']];
+    }
+
+    if (!empty($defaults['entity_type_id']) && !empty($defaults['bundle']) && !empty($defaults['view_mode_name'])) {
+      return $defaults['entity_type_id'] . '.' . $defaults['bundle'] . '.' . $defaults['view_mode_name'];
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSectionListFromId($id) {
+    if (strpos($id, '.') === FALSE) {
+      throw new \InvalidArgumentException(sprintf('The "%s" ID for the "%s" section storage type is invalid', $id, $this->getStorageType()));
+    }
+
+    $storage = $this->entityTypeManager->getStorage('entity_view_display');
+    // If the display does not exist, create a new one.
+    if (!$display = $storage->load($id)) {
+      list($entity_type_id, $bundle, $view_mode) = explode('.', $id, 3);
+      $display = $storage->create([
+        'targetEntityType' => $entity_type_id,
+        'bundle' => $bundle,
+        'mode' => $view_mode,
+        'status' => TRUE,
+      ]);
+    }
+    return $display;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContexts() {
+    $display = $this->getDisplay();
+    $entity = $this->sampleEntityGenerator->get($display->getTargetEntityTypeId(), $display->getTargetBundle());
+    $context_label = new TranslatableMarkup('@entity being viewed', ['@entity' => $entity->getEntityType()->getLabel()]);
+
+    // @todo Use EntityContextDefinition after resolving
+    //   https://www.drupal.org/node/2932462.
+    $contexts = [];
+    $contexts['layout_builder.entity'] = new Context(new ContextDefinition("entity:{$entity->getEntityTypeId()}", $context_label), $entity);
+    return $contexts;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function label() {
+    return $this->getDisplay()->label();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save() {
+    return $this->getDisplay()->save();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function isOverridable() {
+    return $this->getDisplay()->isOverridable();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setOverridable($overridable = TRUE) {
+    $this->getDisplay()->setOverridable($overridable);
+    return $this;
+  }
+
+}

core/modules/layout_builder/src/Plugin/SectionStorage/OverridesSectionStorage.php

@@ -0,0 +1,234 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\SectionStorage;
+
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
+use Drupal\Core\Plugin\Context\Context;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Url;
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Drupal\layout_builder\SectionListInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Defines the 'overrides' section storage type.
+ *
+ * @SectionStorage(
+ *   id = "overrides",
+ * )
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class OverridesSectionStorage extends SectionStorageBase implements ContainerFactoryPluginInterface, OverridesSectionStorageInterface {
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * {@inheritdoc}
+   *
+   * @var \Drupal\layout_builder\SectionListInterface|\Drupal\Core\Field\FieldItemListInterface
+   */
+  protected $sectionList;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+
+    $this->entityTypeManager = $entity_type_manager;
+    $this->entityFieldManager = $entity_field_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+    return new static(
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $container->get('entity_type.manager'),
+      $container->get('entity_field.manager')
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setSectionList(SectionListInterface $section_list) {
+    if (!$section_list instanceof FieldItemListInterface) {
+      throw new \InvalidArgumentException('Overrides expect a field-based section list');
+    }
+
+    return parent::setSectionList($section_list);
+  }
+
+  /**
+   * Gets the entity storing the overrides.
+   *
+   * @return \Drupal\Core\Entity\FieldableEntityInterface
+   *   The entity storing the overrides.
+   */
+  protected function getEntity() {
+    return $this->getSectionList()->getEntity();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStorageId() {
+    $entity = $this->getEntity();
+    return $entity->getEntityTypeId() . '.' . $entity->id();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function extractIdFromRoute($value, $definition, $name, array $defaults) {
+    if (strpos($value, '.') !== FALSE) {
+      return $value;
+    }
+
+    if (isset($defaults['entity_type_id']) && !empty($defaults[$defaults['entity_type_id']])) {
+      $entity_type_id = $defaults['entity_type_id'];
+      $entity_id = $defaults[$entity_type_id];
+      return $entity_type_id . '.' . $entity_id;
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSectionListFromId($id) {
+    if (strpos($id, '.') !== FALSE) {
+      list($entity_type_id, $entity_id) = explode('.', $id, 2);
+      $entity = $this->entityTypeManager->getStorage($entity_type_id)->load($entity_id);
+      if ($entity instanceof FieldableEntityInterface && $entity->hasField('layout_builder__layout')) {
+        return $entity->get('layout_builder__layout');
+      }
+    }
+    throw new \InvalidArgumentException(sprintf('The "%s" ID for the "%s" section storage type is invalid', $id, $this->getStorageType()));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildRoutes(RouteCollection $collection) {
+    foreach ($this->getEntityTypes() as $entity_type_id => $entity_type) {
+      $defaults = [];
+      $defaults['entity_type_id'] = $entity_type_id;
+
+      $requirements = [];
+      if ($this->hasIntegerId($entity_type)) {
+        $requirements[$entity_type_id] = '\d+';
+      }
+
+      $options = [];
+      // Ensure that upcasting is run in the correct order.
+      $options['parameters']['section_storage'] = [];
+      $options['parameters'][$entity_type_id]['type'] = 'entity:' . $entity_type_id;
+
+      $template = $entity_type->getLinkTemplate('canonical') . '/layout';
+      $this->buildLayoutRoutes($collection, $this->getPluginDefinition(), $template, $defaults, $requirements, $options, $entity_type_id);
+    }
+  }
+
+  /**
+   * Determines if this entity type's ID is stored as an integer.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeInterface $entity_type
+   *   An entity type.
+   *
+   * @return bool
+   *   TRUE if this entity type's ID key is always an integer, FALSE otherwise.
+   */
+  protected function hasIntegerId(EntityTypeInterface $entity_type) {
+    $field_storage_definitions = $this->entityFieldManager->getFieldStorageDefinitions($entity_type->id());
+    return $field_storage_definitions[$entity_type->getKey('id')]->getType() === 'integer';
+  }
+
+  /**
+   * Returns an array of relevant entity types.
+   *
+   * @return \Drupal\Core\Entity\EntityTypeInterface[]
+   *   An array of entity types.
+   */
+  protected function getEntityTypes() {
+    return array_filter($this->entityTypeManager->getDefinitions(), function (EntityTypeInterface $entity_type) {
+      return $entity_type->entityClassImplements(FieldableEntityInterface::class) && $entity_type->hasViewBuilderClass() && $entity_type->hasLinkTemplate('canonical');
+    });
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getDefaultSectionStorage() {
+    return LayoutBuilderEntityViewDisplay::collectRenderDisplay($this->getEntity(), 'default');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getRedirectUrl() {
+    return $this->getEntity()->toUrl('canonical');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getLayoutBuilderUrl() {
+    $entity = $this->getEntity();
+    $route_parameters[$entity->getEntityTypeId()] = $entity->id();
+    return Url::fromRoute("layout_builder.{$this->getStorageType()}.{$this->getEntity()->getEntityTypeId()}.view", $route_parameters);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getContexts() {
+    $entity = $this->getEntity();
+    // @todo Use EntityContextDefinition after resolving
+    //   https://www.drupal.org/node/2932462.
+    $contexts['layout_builder.entity'] = new Context(new ContextDefinition("entity:{$entity->getEntityTypeId()}", new TranslatableMarkup('@entity being viewed', ['@entity' => $entity->getEntityType()->getLabel()])), $entity);
+    return $contexts;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function label() {
+    return $this->getEntity()->label();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save() {
+    return $this->getEntity()->save();
+  }
+
+}

core/modules/layout_builder/src/Plugin/SectionStorage/SectionStorageBase.php

@@ -0,0 +1,106 @@
+<?php
+
+namespace Drupal\layout_builder\Plugin\SectionStorage;
+
+use Drupal\Core\Plugin\PluginBase;
+use Drupal\layout_builder\Routing\LayoutBuilderRoutesTrait;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionListInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides a base class for Section Storage types.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+abstract class SectionStorageBase extends PluginBase implements SectionStorageInterface {
+
+  use LayoutBuilderRoutesTrait;
+
+  /**
+   * The section storage instance.
+   *
+   * @var \Drupal\layout_builder\SectionListInterface|null
+   */
+  protected $sectionList;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setSectionList(SectionListInterface $section_list) {
+    $this->sectionList = $section_list;
+    return $this;
+  }
+
+  /**
+   * Gets the section list.
+   *
+   * @return \Drupal\layout_builder\SectionListInterface
+   *   The section list.
+   *
+   * @throws \RuntimeException
+   *   Thrown if ::setSectionList() is not called first.
+   */
+  protected function getSectionList() {
+    if (!$this->sectionList) {
+      throw new \RuntimeException(sprintf('%s::setSectionList() must be called first', static::class));
+    }
+    return $this->sectionList;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getStorageType() {
+    return $this->getPluginId();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function count() {
+    return $this->getSectionList()->count();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSections() {
+    return $this->getSectionList()->getSections();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSection($delta) {
+    return $this->getSectionList()->getSection($delta);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function appendSection(Section $section) {
+    $this->getSectionList()->appendSection($section);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function insertSection($delta, Section $section) {
+    $this->getSectionList()->insertSection($delta, $section);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeSection($delta) {
+    $this->getSectionList()->removeSection($delta);
+    return $this;
+  }
+
+}

core/modules/layout_builder/src/Routing/LayoutBuilderRouteEnhancer.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Drupal\layout_builder\Routing;
+
+use Drupal\Core\Routing\EnhancerInterface;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Converts the query parameter for layout rebuild status to a route default.
+ *
+ * @internal
+ */
+class LayoutBuilderRouteEnhancer implements EnhancerInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function enhance(array $defaults, Request $request) {
+    $route = $defaults[RouteObjectInterface::ROUTE_OBJECT];
+    if ($route->hasOption('_layout_builder')) {
+      $defaults['is_rebuilding'] = (bool) $request->query->get('layout_is_rebuilding', FALSE);
+    }
+    return $defaults;
+  }
+
+}

core/modules/layout_builder/src/Routing/LayoutBuilderRoutes.php

@@ -0,0 +1,56 @@
+<?php
+
+namespace Drupal\layout_builder\Routing;
+
+use Drupal\Core\Routing\RouteBuildEvent;
+use Drupal\Core\Routing\RoutingEvents;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * Provides routes for the Layout Builder UI.
+ *
+ * @internal
+ */
+class LayoutBuilderRoutes implements EventSubscriberInterface {
+
+  /**
+   * The section storage manager.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   */
+  protected $sectionStorageManager;
+
+  /**
+   * Constructs a new LayoutBuilderRoutes.
+   *
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface $section_storage_manager
+   *   The section storage manager.
+   */
+  public function __construct(SectionStorageManagerInterface $section_storage_manager) {
+    $this->sectionStorageManager = $section_storage_manager;
+  }
+
+  /**
+   * Alters existing routes for a specific collection.
+   *
+   * @param \Drupal\Core\Routing\RouteBuildEvent $event
+   *   The route build event.
+   */
+  public function onAlterRoutes(RouteBuildEvent $event) {
+    $collection = $event->getRouteCollection();
+    foreach ($this->sectionStorageManager->getDefinitions() as $plugin_id => $definition) {
+      $this->sectionStorageManager->loadEmpty($plugin_id)->buildRoutes($collection);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    // Run after \Drupal\field_ui\Routing\RouteSubscriber.
+    $events[RoutingEvents::ALTER] = ['onAlterRoutes', -110];
+    return $events;
+  }
+
+}

core/modules/layout_builder/src/Routing/LayoutBuilderRoutesTrait.php

@@ -0,0 +1,97 @@
+<?php
+
+namespace Drupal\layout_builder\Routing;
+
+use Drupal\Component\Utility\NestedArray;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageDefinition;
+use Symfony\Component\Routing\Route;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Provides a trait for building routes for a Layout Builder UI.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+trait LayoutBuilderRoutesTrait {
+
+  /**
+   * Builds the layout routes for the given values.
+   *
+   * @param \Symfony\Component\Routing\RouteCollection $collection
+   *   The route collection.
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageDefinition $definition
+   *   The definition of the section storage.
+   * @param string $path
+   *   The path patten for the routes.
+   * @param array $defaults
+   *   (optional) An array of default parameter values.
+   * @param array $requirements
+   *   (optional) An array of requirements for parameters.
+   * @param array $options
+   *   (optional) An array of options.
+   * @param string $route_name_prefix
+   *   (optional) The prefix to use for the route name.
+   */
+  protected function buildLayoutRoutes(RouteCollection $collection, SectionStorageDefinition $definition, $path, array $defaults = [], array $requirements = [], array $options = [], $route_name_prefix = '') {
+    $type = $definition->id();
+    $defaults['section_storage_type'] = $type;
+    // Provide an empty value to allow the section storage to be upcast.
+    $defaults['section_storage'] = '';
+    // Trigger the layout builder access check.
+    $requirements['_has_layout_section'] = 'true';
+    // Trigger the layout builder RouteEnhancer.
+    $options['_layout_builder'] = TRUE;
+    // Trigger the layout builder param converter.
+    $parameters['section_storage']['layout_builder_tempstore'] = TRUE;
+    // Merge the passed in options in after Layout Builder's parameters.
+    $options = NestedArray::mergeDeep(['parameters' => $parameters], $options);
+
+    if ($route_name_prefix) {
+      $route_name_prefix = "layout_builder.$type.$route_name_prefix";
+    }
+    else {
+      $route_name_prefix = "layout_builder.$type";
+    }
+
+    $main_defaults = $defaults;
+    $main_defaults['is_rebuilding'] = FALSE;
+    $main_defaults['_controller'] = '\Drupal\layout_builder\Controller\LayoutBuilderController::layout';
+    $main_defaults['_title_callback'] = '\Drupal\layout_builder\Controller\LayoutBuilderController::title';
+    $route = (new Route($path))
+      ->setDefaults($main_defaults)
+      ->setRequirements($requirements)
+      ->setOptions($options);
+    $collection->add("$route_name_prefix.view", $route);
+
+    $save_defaults = $defaults;
+    $save_defaults['_controller'] = '\Drupal\layout_builder\Controller\LayoutBuilderController::saveLayout';
+    $route = (new Route("$path/save"))
+      ->setDefaults($save_defaults)
+      ->setRequirements($requirements)
+      ->setOptions($options);
+    $collection->add("$route_name_prefix.save", $route);
+
+    $cancel_defaults = $defaults;
+    $cancel_defaults['_controller'] = '\Drupal\layout_builder\Controller\LayoutBuilderController::cancelLayout';
+    $route = (new Route("$path/cancel"))
+      ->setDefaults($cancel_defaults)
+      ->setRequirements($requirements)
+      ->setOptions($options);
+    $collection->add("$route_name_prefix.cancel", $route);
+
+    if (is_subclass_of($definition->getClass(), OverridesSectionStorageInterface::class)) {
+      $revert_defaults = $defaults;
+      $revert_defaults['_form'] = '\Drupal\layout_builder\Form\RevertOverridesForm';
+      $route = (new Route("$path/revert"))
+        ->setDefaults($revert_defaults)
+        ->setRequirements($requirements)
+        ->setOptions($options);
+      $collection->add("$route_name_prefix.revert", $route);
+    }
+  }
+
+}

core/modules/layout_builder/src/Routing/LayoutTempstoreParamConverter.php

@@ -0,0 +1,63 @@
+<?php
+
+namespace Drupal\layout_builder\Routing;
+
+use Drupal\Core\ParamConverter\ParamConverterInterface;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Symfony\Component\Routing\Route;
+
+/**
+ * Loads the section storage from the layout tempstore.
+ *
+ * @internal
+ */
+class LayoutTempstoreParamConverter implements ParamConverterInterface {
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The section storage manager.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   */
+  protected $sectionStorageManager;
+
+  /**
+   * Constructs a new LayoutTempstoreParamConverter.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The layout tempstore repository.
+   * @param \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface $section_storage_manager
+   *   The section storage manager.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, SectionStorageManagerInterface $section_storage_manager) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->sectionStorageManager = $section_storage_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function convert($value, $definition, $name, array $defaults) {
+    if (isset($defaults['section_storage_type']) && $this->sectionStorageManager->hasDefinition($defaults['section_storage_type'])) {
+      if ($section_storage = $this->sectionStorageManager->loadFromRoute($defaults['section_storage_type'], $value, $definition, $name, $defaults)) {
+        // Pass the plugin through the tempstore repository.
+        return $this->layoutTempstoreRepository->get($section_storage);
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies($definition, $name, Route $route) {
+    return !empty($definition['layout_builder_tempstore']);
+  }
+
+}

core/modules/layout_builder/src/Section.php

@@ -0,0 +1,341 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Provides a domain object for layout sections.
+ *
+ * A section consists of three parts:
+ * - The layout plugin ID for the layout applied to the section (for example,
+ *   'layout_onecol').
+ * - An array of settings for the layout plugin.
+ * - An array of components that can be rendered in the section.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ *
+ * @see \Drupal\Core\Layout\LayoutDefinition
+ * @see \Drupal\layout_builder\SectionComponent
+ *
+ * @todo Determine whether an interface will be provided for this in
+ *   https://www.drupal.org/project/drupal/issues/2930334.
+ */
+class Section {
+
+  /**
+   * The layout plugin ID.
+   *
+   * @var string
+   */
+  protected $layoutId;
+
+  /**
+   * The layout plugin settings.
+   *
+   * @var array
+   */
+  protected $layoutSettings = [];
+
+  /**
+   * An array of components, keyed by UUID.
+   *
+   * @var \Drupal\layout_builder\SectionComponent[]
+   */
+  protected $components = [];
+
+  /**
+   * Constructs a new Section.
+   *
+   * @param string $layout_id
+   *   The layout plugin ID.
+   * @param array $layout_settings
+   *   (optional) The layout plugin settings.
+   * @param \Drupal\layout_builder\SectionComponent[] $components
+   *   (optional) The components.
+   */
+  public function __construct($layout_id, array $layout_settings = [], array $components = []) {
+    $this->layoutId = $layout_id;
+    $this->layoutSettings = $layout_settings;
+    foreach ($components as $component) {
+      $this->setComponent($component);
+    }
+  }
+
+  /**
+   * Returns the renderable array for this section.
+   *
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   An array of available contexts.
+   * @param bool $in_preview
+   *   TRUE if the section is being previewed, FALSE otherwise.
+   *
+   * @return array
+   *   A renderable array representing the content of the section.
+   */
+  public function toRenderArray(array $contexts = [], $in_preview = FALSE) {
+    $regions = [];
+    foreach ($this->getComponents() as $component) {
+      if ($output = $component->toRenderArray($contexts, $in_preview)) {
+        $regions[$component->getRegion()][$component->getUuid()] = $output;
+      }
+    }
+
+    return $this->getLayout()->build($regions);
+  }
+
+  /**
+   * Gets the layout plugin for this section.
+   *
+   * @return \Drupal\Core\Layout\LayoutInterface
+   *   The layout plugin.
+   */
+  public function getLayout() {
+    return $this->layoutPluginManager()->createInstance($this->getLayoutId(), $this->getLayoutSettings());
+  }
+
+  /**
+   * Gets the layout plugin ID for this section.
+   *
+   * @return string
+   *   The layout plugin ID.
+   *
+   * @internal
+   *   This method should only be used by code responsible for storing the data.
+   */
+  public function getLayoutId() {
+    return $this->layoutId;
+  }
+
+  /**
+   * Gets the layout plugin settings for this section.
+   *
+   * @return mixed[]
+   *   The layout plugin settings.
+   *
+   * @internal
+   *   This method should only be used by code responsible for storing the data.
+   */
+  public function getLayoutSettings() {
+    return $this->layoutSettings;
+  }
+
+  /**
+   * Sets the layout plugin settings for this section.
+   *
+   * @param mixed[] $layout_settings
+   *   The layout plugin settings.
+   *
+   * @return $this
+   */
+  public function setLayoutSettings(array $layout_settings) {
+    $this->layoutSettings = $layout_settings;
+    return $this;
+  }
+
+  /**
+   * Gets the default region.
+   *
+   * @return string
+   *   The machine-readable name of the default region.
+   */
+  public function getDefaultRegion() {
+    return $this->layoutPluginManager()->getDefinition($this->getLayoutId())->getDefaultRegion();
+  }
+
+  /**
+   * Returns the components of the section.
+   *
+   * @return \Drupal\layout_builder\SectionComponent[]
+   *   The components.
+   */
+  public function getComponents() {
+    return $this->components;
+  }
+
+  /**
+   * Gets the component for a given UUID.
+   *
+   * @param string $uuid
+   *   The UUID of the component to retrieve.
+   *
+   * @return \Drupal\layout_builder\SectionComponent
+   *   The component.
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown when the expected UUID does not exist.
+   */
+  public function getComponent($uuid) {
+    if (!isset($this->components[$uuid])) {
+      throw new \InvalidArgumentException(sprintf('Invalid UUID "%s"', $uuid));
+    }
+
+    return $this->components[$uuid];
+  }
+
+  /**
+   * Helper method to set a component.
+   *
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The component.
+   *
+   * @return $this
+   */
+  protected function setComponent(SectionComponent $component) {
+    $this->components[$component->getUuid()] = $component;
+    return $this;
+  }
+
+  /**
+   * Removes a given component from a region.
+   *
+   * @param string $uuid
+   *   The UUID of the component to remove.
+   *
+   * @return $this
+   */
+  public function removeComponent($uuid) {
+    unset($this->components[$uuid]);
+    return $this;
+  }
+
+  /**
+   * Appends a component to the end of a region.
+   *
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The component being appended.
+   *
+   * @return $this
+   */
+  public function appendComponent(SectionComponent $component) {
+    $component->setWeight($this->getNextHighestWeight($component->getRegion()));
+    $this->setComponent($component);
+    return $this;
+  }
+
+  /**
+   * Returns the next highest weight of the component in a region.
+   *
+   * @param string $region
+   *   The region name.
+   *
+   * @return int
+   *   A number higher than the highest weight of the component in the region.
+   */
+  protected function getNextHighestWeight($region) {
+    $components = $this->getComponentsByRegion($region);
+    $weights = array_map(function (SectionComponent $component) {
+      return $component->getWeight();
+    }, $components);
+    return $weights ? max($weights) + 1 : 0;
+  }
+
+  /**
+   * Gets the components for a specific region.
+   *
+   * @param string $region
+   *   The region name.
+   *
+   * @return \Drupal\layout_builder\SectionComponent[]
+   *   An array of components in the specified region, sorted by weight.
+   */
+  protected function getComponentsByRegion($region) {
+    $components = array_filter($this->getComponents(), function (SectionComponent $component) use ($region) {
+      return $component->getRegion() === $region;
+    });
+    uasort($components, function (SectionComponent $a, SectionComponent $b) {
+      return $a->getWeight() > $b->getWeight() ? 1 : -1;
+    });
+    return $components;
+  }
+
+  /**
+   * Inserts a component after a specified existing component.
+   *
+   * @param string $preceding_uuid
+   *   The UUID of the existing component to insert after.
+   * @param \Drupal\layout_builder\SectionComponent $component
+   *   The component being inserted.
+   *
+   * @return $this
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown when the expected UUID does not exist.
+   */
+  public function insertAfterComponent($preceding_uuid, SectionComponent $component) {
+    // Find the delta of the specified UUID.
+    $uuids = array_keys($this->getComponentsByRegion($component->getRegion()));
+    $delta = array_search($preceding_uuid, $uuids, TRUE);
+    if ($delta === FALSE) {
+      throw new \InvalidArgumentException(sprintf('Invalid preceding UUID "%s"', $preceding_uuid));
+    }
+    return $this->insertComponent($delta + 1, $component);
+  }
+
+  /**
+   * Inserts a component at a specified delta.
+   *
+   * @param int $delta
+   *   The zero-based delta in which to insert the component.
+   * @param \Drupal\layout_builder\SectionComponent $new_component
+   *   The component being inserted.
+   *
+   * @return $this
+   *
+   * @throws \OutOfBoundsException
+   *   Thrown when the specified delta is invalid.
+   */
+  public function insertComponent($delta, SectionComponent $new_component) {
+    $components = $this->getComponentsByRegion($new_component->getRegion());
+    $count = count($components);
+    if ($delta > $count) {
+      throw new \OutOfBoundsException(sprintf('Invalid delta "%s" for the "%s" component', $delta, $new_component->getUuid()));
+    }
+
+    // If the delta is the end of the list, append the component instead.
+    if ($delta === $count) {
+      return $this->appendComponent($new_component);
+    }
+
+    // Find the weight of the component that exists at the specified delta.
+    $weight = array_values($components)[$delta]->getWeight();
+    $this->setComponent($new_component->setWeight($weight++));
+
+    // Increase the weight of every subsequent component.
+    foreach (array_slice($components, $delta) as $component) {
+      $component->setWeight($weight++);
+    }
+    return $this;
+  }
+
+  /**
+   * Wraps the layout plugin manager.
+   *
+   * @return \Drupal\Core\Layout\LayoutPluginManagerInterface
+   *   The layout plugin manager.
+   */
+  protected function layoutPluginManager() {
+    return \Drupal::service('plugin.manager.core.layout');
+  }
+
+  /**
+   * Returns an array representation of the section.
+   *
+   * @internal
+   *   This is intended for use by a storage mechanism for sections.
+   *
+   * @return array
+   *   An array representation of the section component.
+   */
+  public function toArray() {
+    return [
+      'layout_id' => $this->getLayoutId(),
+      'layout_settings' => $this->getLayoutSettings(),
+      'components' => array_map(function (SectionComponent $component) {
+        return $component->toArray();
+      }, $this->getComponents()),
+    ];
+  }
+
+}

core/modules/layout_builder/src/SectionComponent.php

@@ -0,0 +1,311 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Component\Plugin\Exception\PluginException;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent;
+
+/**
+ * Provides a value object for a section component.
+ *
+ * A component represents the smallest part of a layout (for example, a block).
+ * Components wrap a renderable plugin, currently using
+ * \Drupal\Core\Block\BlockPluginInterface, and contain the layout region
+ * within the section layout where the component will be rendered.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ *
+ * @see \Drupal\Core\Layout\LayoutDefinition
+ * @see \Drupal\layout_builder\Section
+ * @see \Drupal\layout_builder\SectionStorageInterface
+ *
+ * @todo Determine whether to retain the name 'component' in
+ *   https://www.drupal.org/project/drupal/issues/2929783.
+ * @todo Determine whether an interface will be provided for this in
+ *   https://www.drupal.org/project/drupal/issues/2930334.
+ */
+class SectionComponent {
+
+  /**
+   * The UUID of the component.
+   *
+   * @var string
+   */
+  protected $uuid;
+
+  /**
+   * The region the component is placed in.
+   *
+   * @var string
+   */
+  protected $region;
+
+  /**
+   * An array of plugin configuration.
+   *
+   * @var mixed[]
+   */
+  protected $configuration;
+
+  /**
+   * The weight of the component.
+   *
+   * @var int
+   */
+  protected $weight = 0;
+
+  /**
+   * Any additional properties and values.
+   *
+   * @var mixed[]
+   */
+  protected $additional = [];
+
+  /**
+   * Constructs a new SectionComponent.
+   *
+   * @param string $uuid
+   *   The UUID.
+   * @param string $region
+   *   The region.
+   * @param mixed[] $configuration
+   *   The plugin configuration.
+   * @param mixed[] $additional
+   *   An additional values.
+   */
+  public function __construct($uuid, $region, array $configuration = [], array $additional = []) {
+    $this->uuid = $uuid;
+    $this->region = $region;
+    $this->configuration = $configuration;
+    $this->additional = $additional;
+  }
+
+  /**
+   * Returns the renderable array for this component.
+   *
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   An array of available contexts.
+   * @param bool $in_preview
+   *   TRUE if the component is being previewed, FALSE otherwise.
+   *
+   * @return array
+   *   A renderable array representing the content of the component.
+   */
+  public function toRenderArray(array $contexts = [], $in_preview = FALSE) {
+    $event = new SectionComponentBuildRenderArrayEvent($this, $contexts, $in_preview);
+    $this->eventDispatcher()->dispatch(LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY, $event);
+    $output = $event->getBuild();
+    $event->getCacheableMetadata()->applyTo($output);
+    return $output;
+  }
+
+  /**
+   * Gets any arbitrary property for the component.
+   *
+   * @param string $property
+   *   The property to retrieve.
+   *
+   * @return mixed
+   *   The value for that property, or NULL if the property does not exist.
+   */
+  public function get($property) {
+    if (property_exists($this, $property)) {
+      $value = isset($this->{$property}) ? $this->{$property} : NULL;
+    }
+    else {
+      $value = isset($this->additional[$property]) ? $this->additional[$property] : NULL;
+    }
+    return $value;
+  }
+
+  /**
+   * Sets a value to an arbitrary property for the component.
+   *
+   * @param string $property
+   *   The property to use for the value.
+   * @param mixed $value
+   *   The value to set.
+   *
+   * @return $this
+   */
+  public function set($property, $value) {
+    if (property_exists($this, $property)) {
+      $this->{$property} = $value;
+    }
+    else {
+      $this->additional[$property] = $value;
+    }
+    return $this;
+  }
+
+  /**
+   * Gets the region for the component.
+   *
+   * @return string
+   *   The region.
+   */
+  public function getRegion() {
+    return $this->region;
+  }
+
+  /**
+   * Sets the region for the component.
+   *
+   * @param string $region
+   *   The region.
+   *
+   * @return $this
+   */
+  public function setRegion($region) {
+    $this->region = $region;
+    return $this;
+  }
+
+  /**
+   * Gets the weight of the component.
+   *
+   * @return int
+   *   The zero-based weight of the component.
+   *
+   * @throws \UnexpectedValueException
+   *   Thrown if the weight was never set.
+   */
+  public function getWeight() {
+    return $this->weight;
+  }
+
+  /**
+   * Sets the weight of the component.
+   *
+   * @param int $weight
+   *   The zero-based weight of the component.
+   *
+   * @return $this
+   */
+  public function setWeight($weight) {
+    $this->weight = $weight;
+    return $this;
+  }
+
+  /**
+   * Gets the component plugin configuration.
+   *
+   * @return mixed[]
+   *   The component plugin configuration.
+   */
+  protected function getConfiguration() {
+    return $this->configuration;
+  }
+
+  /**
+   * Sets the plugin configuration.
+   *
+   * @param mixed[] $configuration
+   *   The plugin configuration.
+   *
+   * @return $this
+   */
+  public function setConfiguration(array $configuration) {
+    $this->configuration = $configuration;
+    return $this;
+  }
+
+  /**
+   * Gets the plugin ID.
+   *
+   * @return string
+   *   The plugin ID.
+   *
+   * @throws \Drupal\Component\Plugin\Exception\PluginException
+   *   Thrown if the plugin ID cannot be found.
+   */
+  public function getPluginId() {
+    if (empty($this->configuration['id'])) {
+      throw new PluginException(sprintf('No plugin ID specified for component with "%s" UUID', $this->uuid));
+    }
+    return $this->configuration['id'];
+  }
+
+  /**
+   * Gets the UUID for this component.
+   *
+   * @return string
+   *   The UUID.
+   */
+  public function getUuid() {
+    return $this->uuid;
+  }
+
+  /**
+   * Gets the plugin for this component.
+   *
+   * @param \Drupal\Core\Plugin\Context\ContextInterface[] $contexts
+   *   An array of contexts to set on the plugin.
+   *
+   * @return \Drupal\Component\Plugin\PluginInspectionInterface
+   *   The plugin.
+   */
+  public function getPlugin(array $contexts = []) {
+    $plugin = $this->pluginManager()->createInstance($this->getPluginId(), $this->getConfiguration());
+    if ($contexts && $plugin instanceof ContextAwarePluginInterface) {
+      $this->contextHandler()->applyContextMapping($plugin, $contexts);
+    }
+    return $plugin;
+  }
+
+  /**
+   * Wraps the component plugin manager.
+   *
+   * @return \Drupal\Core\Block\BlockManagerInterface
+   *   The plugin manager.
+   */
+  protected function pluginManager() {
+    // @todo Figure out the best way to unify fields and blocks and components
+    //   in https://www.drupal.org/node/1875974.
+    return \Drupal::service('plugin.manager.block');
+  }
+
+  /**
+   * Wraps the context handler.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextHandlerInterface
+   *   The context handler.
+   */
+  protected function contextHandler() {
+    return \Drupal::service('context.handler');
+  }
+
+  /**
+   * Wraps the event dispatcher.
+   *
+   * @return \Symfony\Component\EventDispatcher\EventDispatcherInterface
+   *   The event dispatcher.
+   */
+  protected function eventDispatcher() {
+    return \Drupal::service('event_dispatcher');
+  }
+
+  /**
+   * Returns an array representation of the section component.
+   *
+   * @internal
+   *   This is intended for use by a storage mechanism for section components.
+   *
+   * @return array
+   *   An array representation of the section component.
+   */
+  public function toArray() {
+    return [
+      'uuid' => $this->getUuid(),
+      'region' => $this->getRegion(),
+      'configuration' => $this->getConfiguration(),
+      'additional' => $this->additional,
+      'weight' => $this->getWeight(),
+    ];
+  }
+
+}

core/modules/layout_builder/src/SectionListInterface.php

@@ -0,0 +1,74 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+/**
+ * Defines the interface for an object that stores layout sections.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ *
+ * @see \Drupal\layout_builder\Section
+ */
+interface SectionListInterface extends \Countable {
+
+  /**
+   * Gets the layout sections.
+   *
+   * @return \Drupal\layout_builder\Section[]
+   *   A sequentially and numerically keyed array of section objects.
+   */
+  public function getSections();
+
+  /**
+   * Gets a domain object for the layout section.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   *
+   * @return \Drupal\layout_builder\Section
+   *   The layout section.
+   */
+  public function getSection($delta);
+
+  /**
+   * Appends a new section to the end of the list.
+   *
+   * @param \Drupal\layout_builder\Section $section
+   *   The section to append.
+   *
+   * @return $this
+   */
+  public function appendSection(Section $section);
+
+  /**
+   * Inserts a new section at a given delta.
+   *
+   * If a section exists at the given index, the section at that position and
+   * others after it are shifted backward.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   * @param \Drupal\layout_builder\Section $section
+   *   The section to insert.
+   *
+   * @return $this
+   */
+  public function insertSection($delta, Section $section);
+
+  /**
+   * Removes the section at the given delta.
+   *
+   * As sections are stored sequentially and numerically this will re-key every
+   * subsequent section, shifting them forward.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   *
+   * @return $this
+   */
+  public function removeSection($delta);
+
+}

core/modules/layout_builder/src/SectionStorage/SectionStorageDefinition.php

@@ -0,0 +1,75 @@
+<?php
+
+namespace Drupal\layout_builder\SectionStorage;
+
+use Drupal\Component\Plugin\Definition\PluginDefinition;
+
+/**
+ * Provides section storage type plugin definition.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class SectionStorageDefinition extends PluginDefinition {
+
+  /**
+   * Any additional properties and values.
+   *
+   * @var array
+   */
+  protected $additional = [];
+
+  /**
+   * LayoutDefinition constructor.
+   *
+   * @param array $definition
+   *   An array of values from the annotation.
+   */
+  public function __construct(array $definition = []) {
+    foreach ($definition as $property => $value) {
+      $this->set($property, $value);
+    }
+  }
+
+  /**
+   * Gets any arbitrary property.
+   *
+   * @param string $property
+   *   The property to retrieve.
+   *
+   * @return mixed
+   *   The value for that property, or NULL if the property does not exist.
+   */
+  public function get($property) {
+    if (property_exists($this, $property)) {
+      $value = isset($this->{$property}) ? $this->{$property} : NULL;
+    }
+    else {
+      $value = isset($this->additional[$property]) ? $this->additional[$property] : NULL;
+    }
+    return $value;
+  }
+
+  /**
+   * Sets a value to an arbitrary property.
+   *
+   * @param string $property
+   *   The property to use for the value.
+   * @param mixed $value
+   *   The value to set.
+   *
+   * @return $this
+   */
+  public function set($property, $value) {
+    if (property_exists($this, $property)) {
+      $this->{$property} = $value;
+    }
+    else {
+      $this->additional[$property] = $value;
+    }
+    return $this;
+  }
+
+}

core/modules/layout_builder/src/SectionStorage/SectionStorageManager.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Drupal\layout_builder\SectionStorage;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\DefaultPluginManager;
+use Drupal\layout_builder\Annotation\SectionStorage;
+use Drupal\layout_builder\SectionStorageInterface;
+
+/**
+ * Provides the Section Storage type plugin manager.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+class SectionStorageManager extends DefaultPluginManager implements SectionStorageManagerInterface {
+
+  /**
+   * Constructs a new SectionStorageManager object.
+   *
+   * @param \Traversable $namespaces
+   *   An object that implements \Traversable which contains the root paths
+   *   keyed by the corresponding namespace to look for plugin implementations.
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
+   *   Cache backend instance to use.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler to invoke the alter hook with.
+   */
+  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
+    parent::__construct('Plugin/SectionStorage', $namespaces, $module_handler, SectionStorageInterface::class, SectionStorage::class);
+
+    $this->alterInfo('layout_builder_section_storage');
+    $this->setCacheBackend($cache_backend, 'layout_builder_section_storage_plugins');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function loadEmpty($id) {
+    return $this->createInstance($id);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function loadFromStorageId($type, $id) {
+    /** @var \Drupal\layout_builder\SectionStorageInterface $plugin */
+    $plugin = $this->createInstance($type);
+    return $plugin->setSectionList($plugin->getSectionListFromId($id));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function loadFromRoute($type, $value, $definition, $name, array $defaults) {
+    /** @var \Drupal\layout_builder\SectionStorageInterface $plugin */
+    $plugin = $this->createInstance($type);
+    if ($id = $plugin->extractIdFromRoute($value, $definition, $name, $defaults)) {
+      try {
+        return $plugin->setSectionList($plugin->getSectionListFromId($id));
+      }
+      catch (\InvalidArgumentException $e) {
+        // Intentionally empty.
+      }
+    }
+  }
+
+}

core/modules/layout_builder/src/SectionStorage/SectionStorageManagerInterface.php

@@ -0,0 +1,65 @@
+<?php
+
+namespace Drupal\layout_builder\SectionStorage;
+
+use Drupal\Component\Plugin\Discovery\DiscoveryInterface;
+
+/**
+ * Provides the interface for a plugin manager of section storage types.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+interface SectionStorageManagerInterface extends DiscoveryInterface {
+
+  /**
+   * Loads a section storage with no associated section list.
+   *
+   * @param string $id
+   *   The ID of the section storage being instantiated.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage.
+   */
+  public function loadEmpty($id);
+
+  /**
+   * Loads a section storage populated with an existing section list.
+   *
+   * @param string $type
+   *   The section storage type.
+   * @param string $id
+   *   The section list ID.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage.
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown if the ID is invalid.
+   */
+  public function loadFromStorageId($type, $id);
+
+  /**
+   * Loads a section storage populated with a section list derived from a route.
+   *
+   * @param string $type
+   *   The section storage type.
+   * @param string $value
+   *   The raw value.
+   * @param mixed $definition
+   *   The parameter definition provided in the route options.
+   * @param string $name
+   *   The name of the parameter.
+   * @param array $defaults
+   *   The route defaults array.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface|null
+   *   The section storage if it could be loaded, or NULL otherwise.
+   *
+   * @see \Drupal\Core\ParamConverter\ParamConverterInterface::convert()
+   */
+  public function loadFromRoute($type, $value, $definition, $name, array $defaults);
+
+}

core/modules/layout_builder/src/SectionStorage/SectionStorageTrait.php

@@ -0,0 +1,114 @@
+<?php
+
+namespace Drupal\layout_builder\SectionStorage;
+
+use Drupal\layout_builder\Section;
+
+/**
+ * Provides a trait for storing sections on an object.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+trait SectionStorageTrait {
+
+  /**
+   * Stores the information for all sections.
+   *
+   * Implementations of this method are expected to call array_values() to rekey
+   * the list of sections.
+   *
+   * @param \Drupal\layout_builder\Section[] $sections
+   *   An array of section objects.
+   *
+   * @return $this
+   */
+  abstract protected function setSections(array $sections);
+
+  /**
+   * {@inheritdoc}
+   */
+  public function count() {
+    return count($this->getSections());
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getSection($delta) {
+    if (!$this->hasSection($delta)) {
+      throw new \OutOfBoundsException(sprintf('Invalid delta "%s"', $delta));
+    }
+
+    return $this->getSections()[$delta];
+  }
+
+  /**
+   * Sets the section for the given delta on the display.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   * @param \Drupal\layout_builder\Section $section
+   *   The layout section.
+   *
+   * @return $this
+   */
+  protected function setSection($delta, Section $section) {
+    $sections = $this->getSections();
+    $sections[$delta] = $section;
+    $this->setSections($sections);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function appendSection(Section $section) {
+    $delta = $this->count();
+
+    $this->setSection($delta, $section);
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function insertSection($delta, Section $section) {
+    if ($this->hasSection($delta)) {
+      // @todo Use https://www.drupal.org/node/66183 once resolved.
+      $start = array_slice($this->getSections(), 0, $delta);
+      $end = array_slice($this->getSections(), $delta);
+      $this->setSections(array_merge($start, [$section], $end));
+    }
+    else {
+      $this->appendSection($section);
+    }
+    return $this;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function removeSection($delta) {
+    $sections = $this->getSections();
+    unset($sections[$delta]);
+    $this->setSections($sections);
+    return $this;
+  }
+
+  /**
+   * Indicates if there is a section at the specified delta.
+   *
+   * @param int $delta
+   *   The delta of the section.
+   *
+   * @return bool
+   *   TRUE if there is a section for this delta, FALSE otherwise.
+   */
+  protected function hasSection($delta) {
+    return isset($this->getSections()[$delta]);
+  }
+
+}

core/modules/layout_builder/src/SectionStorageInterface.php

@@ -0,0 +1,141 @@
+<?php
+
+namespace Drupal\layout_builder;
+
+use Drupal\Component\Plugin\PluginInspectionInterface;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * Defines an interface for Section Storage type plugins.
+ *
+ * @internal
+ *   Layout Builder is currently experimental and should only be leveraged by
+ *   experimental modules and development releases of contributed modules.
+ *   See https://www.drupal.org/core/experimental for more information.
+ */
+interface SectionStorageInterface extends SectionListInterface, PluginInspectionInterface {
+
+  /**
+   * Returns an identifier for this storage.
+   *
+   * @return string
+   *   The unique identifier for this storage.
+   */
+  public function getStorageId();
+
+  /**
+   * Returns the type of this storage.
+   *
+   * Used in conjunction with the storage ID.
+   *
+   * @return string
+   *   The type of storage.
+   */
+  public function getStorageType();
+
+  /**
+   * Sets the section list on the storage.
+   *
+   * @param \Drupal\layout_builder\SectionListInterface $section_list
+   *   The section list.
+   *
+   * @return $this
+   *
+   * @internal
+   *   This should only be called during section storage instantiation.
+   */
+  public function setSectionList(SectionListInterface $section_list);
+
+  /**
+   * Derives the section list from the storage ID.
+   *
+   * @param string $id
+   *   The storage ID, see ::getStorageId().
+   *
+   * @return \Drupal\layout_builder\SectionListInterface
+   *   The section list.
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown if the ID is invalid.
+   *
+   * @internal
+   *   This should only be called during section storage instantiation.
+   */
+  public function getSectionListFromId($id);
+
+  /**
+   * Provides the routes needed for Layout Builder UI.
+   *
+   * Allows the plugin to add or alter routes during the route building process.
+   * \Drupal\layout_builder\Routing\LayoutBuilderRoutesTrait is provided for the
+   * typical use case of building a standard Layout Builder UI.
+   *
+   * @param \Symfony\Component\Routing\RouteCollection $collection
+   *   The route collection.
+   *
+   * @see \Drupal\Core\Routing\RoutingEvents::ALTER
+   */
+  public function buildRoutes(RouteCollection $collection);
+
+  /**
+   * Gets the URL used when redirecting away from the Layout Builder UI.
+   *
+   * @return \Drupal\Core\Url
+   *   The URL object.
+   */
+  public function getRedirectUrl();
+
+  /**
+   * Gets the URL used to display the Layout Builder UI.
+   *
+   * @return \Drupal\Core\Url
+   *   The URL object.
+   */
+  public function getLayoutBuilderUrl();
+
+  /**
+   * Configures the plugin based on route values.
+   *
+   * @param mixed $value
+   *   The raw value.
+   * @param mixed $definition
+   *   The parameter definition provided in the route options.
+   * @param string $name
+   *   The name of the parameter.
+   * @param array $defaults
+   *   The route defaults array.
+   *
+   * @return string|null
+   *   The section storage ID if it could be extracted, NULL otherwise.
+   *
+   * @internal
+   *   This should only be called during section storage instantiation.
+   */
+  public function extractIdFromRoute($value, $definition, $name, array $defaults);
+
+  /**
+   * Provides any available contexts for the object using the sections.
+   *
+   * @return \Drupal\Core\Plugin\Context\ContextInterface[]
+   *   The array of context objects.
+   */
+  public function getContexts();
+
+  /**
+   * Gets the label for the object using the sections.
+   *
+   * @return string
+   *   The label, or NULL if there is no label defined.
+   */
+  public function label();
+
+  /**
+   * Saves the sections.
+   *
+   * @return int
+   *   SAVED_NEW or SAVED_UPDATED is returned depending on the operation
+   *   performed.
+   */
+  public function save();
+
+}

core/modules/layout_builder/tests/fixtures/update/section-dependencies.php

@@ -0,0 +1,54 @@
+<?php
+
+/**
+ * @file
+ * Test fixture.
+ */
+
+use Drupal\Core\Database\Database;
+
+$connection = Database::getConnection();
+
+// Update core.extension.
+$extensions = $connection->select('config')
+  ->fields('config', ['data'])
+  ->condition('collection', '')
+  ->condition('name', 'core.extension')
+  ->execute()
+  ->fetchField();
+$extensions = unserialize($extensions);
+$extensions['module']['layout_builder'] = 0;
+$extensions['module']['layout_discovery'] = 0;
+$extensions['module']['layout_test'] = 0;
+$connection->update('config')
+  ->fields([
+    'data' => serialize($extensions),
+    'collection' => '',
+    'name' => 'core.extension',
+  ])
+  ->condition('collection', '')
+  ->condition('name', 'core.extension')
+  ->execute();
+
+// Add a layout plugin with a dependency to an existing entity view display.
+$display = $connection->select('config')
+  ->fields('config', ['data'])
+  ->condition('collection', '')
+  ->condition('name', 'core.entity_view_display.node.article.teaser')
+  ->execute()
+  ->fetchField();
+$display = unserialize($display);
+$display['third_party_settings']['layout_builder']['sections'][] = [
+  'layout_id' => 'layout_test_dependencies_plugin',
+  'layout_settings' => [],
+  'components' => [],
+];
+$connection->update('config')
+  ->fields([
+    'data' => serialize($display),
+    'collection' => '',
+    'name' => 'core.entity_view_display.node.article.teaser',
+  ])
+  ->condition('collection', '')
+  ->condition('name', 'core.entity_view_display.node.article.teaser')
+  ->execute();

core/modules/layout_builder/tests/src/Functional/LayoutBuilderTest.php

@@ -0,0 +1,259 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Functional;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the Layout Builder UI.
+ *
+ * @group layout_builder
+ */
+class LayoutBuilderTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'layout_builder',
+    'layout_test',
+    'block',
+    'node',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    // @todo The Layout Builder UI relies on local tasks; fix in
+    //   https://www.drupal.org/project/drupal/issues/2917777.
+    $this->drupalPlaceBlock('local_tasks_block');
+
+    // Create two nodes.
+    $this->createContentType(['type' => 'bundle_with_section_field']);
+    $this->createNode([
+      'type' => 'bundle_with_section_field',
+      'title' => 'The first node title',
+      'body' => [
+        [
+          'value' => 'The first node body',
+        ],
+      ],
+    ]);
+    $this->createNode([
+      'type' => 'bundle_with_section_field',
+      'title' => 'The second node title',
+      'body' => [
+        [
+          'value' => 'The second node body',
+        ],
+      ],
+    ]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function testLayoutBuilderUi() {
+    $assert_session = $this->assertSession();
+    $page = $this->getSession()->getPage();
+
+    $this->drupalLogin($this->drupalCreateUser([
+      'configure any layout',
+      'administer node display',
+      'administer node fields',
+    ]));
+
+    $this->drupalGet('node/1');
+    $assert_session->pageTextContains('The first node body');
+    $assert_session->pageTextNotContains('Powered by Drupal');
+    $assert_session->linkNotExists('Layout');
+
+    $field_ui_prefix = 'admin/structure/types/manage/bundle_with_section_field';
+
+    // From the manage display page, go to manage the layout.
+    $this->drupalGet("$field_ui_prefix/display/default");
+    $assert_session->linkExists('Manage layout');
+    $this->clickLink('Manage layout');
+    $assert_session->addressEquals("$field_ui_prefix/display-layout/default");
+    // The body field is present.
+    $assert_session->elementExists('css', '.field--name-body');
+
+    // Add a new block.
+    $assert_session->linkExists('Add Block');
+    $this->clickLink('Add Block');
+    $assert_session->linkExists('Powered by Drupal');
+    $this->clickLink('Powered by Drupal');
+    $page->fillField('settings[label]', 'This is the label');
+    $page->checkField('settings[label_display]');
+    $page->pressButton('Add Block');
+    $assert_session->pageTextContains('Powered by Drupal');
+    $assert_session->pageTextContains('This is the label');
+    $assert_session->addressEquals("$field_ui_prefix/display-layout/default");
+
+    // Save the defaults.
+    $assert_session->linkExists('Save Layout');
+    $this->clickLink('Save Layout');
+    $assert_session->addressEquals("$field_ui_prefix/display/default");
+
+    // The node uses the defaults, no overrides available.
+    $this->drupalGet('node/1');
+    $assert_session->pageTextContains('The first node body');
+    $assert_session->pageTextContains('Powered by Drupal');
+    $assert_session->linkNotExists('Layout');
+
+    // Enable overrides.
+    $this->drupalPostForm("$field_ui_prefix/display/default", ['layout[allow_custom]' => TRUE], 'Save');
+    $this->drupalGet('node/1');
+
+    // Remove the section from the defaults.
+    $assert_session->linkExists('Layout');
+    $this->clickLink('Layout');
+    $assert_session->linkExists('Remove section');
+    $this->clickLink('Remove section');
+    $page->pressButton('Remove');
+
+    // Add a new section.
+    $this->clickLink('Add Section');
+    $assert_session->linkExists('Two column');
+    $this->clickLink('Two column');
+    $assert_session->linkExists('Save Layout');
+    $this->clickLink('Save Layout');
+    $assert_session->pageTextNotContains('The first node body');
+    $assert_session->pageTextNotContains('Powered by Drupal');
+
+    // Assert that overrides cannot be turned off while overrides exist.
+    $this->drupalGet("$field_ui_prefix/display/default");
+    $assert_session->fieldDisabled('layout[allow_custom]');
+
+    // Alter the defaults.
+    $this->drupalGet("$field_ui_prefix/display-layout/default");
+    $assert_session->linkExists('Add Block');
+    $this->clickLink('Add Block');
+    $assert_session->linkExists('Title');
+    $this->clickLink('Title');
+    $page->pressButton('Add Block');
+    // The title field is present.
+    $assert_session->elementExists('css', '.field--name-title');
+    $this->clickLink('Save Layout');
+
+    // View the other node, which is still using the defaults.
+    $this->drupalGet('node/2');
+    $assert_session->pageTextContains('The second node title');
+    $assert_session->pageTextContains('The second node body');
+    $assert_session->pageTextContains('Powered by Drupal');
+
+    // The overridden node does not pick up the changes to defaults.
+    $this->drupalGet('node/1');
+    $assert_session->elementNotExists('css', '.field--name-title');
+    $assert_session->pageTextNotContains('The first node body');
+    $assert_session->pageTextNotContains('Powered by Drupal');
+    $assert_session->linkExists('Layout');
+
+    // Reverting the override returns it to the defaults.
+    $this->clickLink('Layout');
+    $assert_session->linkExists('Revert to defaults');
+    $this->clickLink('Revert to defaults');
+    $page->pressButton('Revert');
+    $assert_session->pageTextContains('The layout has been reverted back to defaults.');
+    $assert_session->elementExists('css', '.field--name-title');
+    $assert_session->pageTextContains('The first node body');
+    $assert_session->pageTextContains('Powered by Drupal');
+
+    // Assert that overrides can be turned off now that all overrides are gone.
+    $this->drupalPostForm("$field_ui_prefix/display/default", ['layout[allow_custom]' => FALSE], 'Save');
+    $this->drupalGet('node/1');
+    $assert_session->linkNotExists('Layout');
+
+    // Add a new field.
+    $edit = [
+      'new_storage_type' => 'string',
+      'label' => 'My text field',
+      'field_name' => 'my_text',
+    ];
+    $this->drupalPostForm("$field_ui_prefix/fields/add-field", $edit, 'Save and continue');
+    $page->pressButton('Save field settings');
+    $page->pressButton('Save settings');
+    $this->drupalGet("$field_ui_prefix/display-layout/default");
+    $assert_session->pageTextContains('My text field');
+    $assert_session->elementExists('css', '.field--name-field-my-text');
+
+    // Delete the field.
+    $this->drupalPostForm("$field_ui_prefix/fields/node.bundle_with_section_field.field_my_text/delete", [], 'Delete');
+    $this->drupalGet("$field_ui_prefix/display-layout/default");
+    $assert_session->pageTextNotContains('My text field');
+    $assert_session->elementNotExists('css', '.field--name-field-my-text');
+  }
+
+  /**
+   * Tests that component's dependencies are respected during removal.
+   */
+  public function testPluginDependencies() {
+    $assert_session = $this->assertSession();
+    $page = $this->getSession()->getPage();
+
+    $this->container->get('module_installer')->install(['menu_ui']);
+    $this->drupalLogin($this->drupalCreateUser([
+      'configure any layout',
+      'administer node display',
+      'administer menu',
+    ]));
+
+    // Create a new menu.
+    $this->drupalGet('admin/structure/menu/add');
+    $page->fillField('label', 'My Menu');
+    $page->fillField('id', 'mymenu');
+    $page->pressButton('Save');
+    $this->drupalGet('admin/structure/menu/add');
+    $page->fillField('label', 'My Menu');
+    $page->fillField('id', 'myothermenu');
+    $page->pressButton('Save');
+
+    $this->drupalGet('admin/structure/types/manage/bundle_with_section_field/display-layout/default');
+    $assert_session->linkExists('Add Section');
+    $this->clickLink('Add Section');
+    $assert_session->linkExists('Layout plugin (with dependencies)');
+    $this->clickLink('Layout plugin (with dependencies)');
+    $assert_session->elementExists('css', '.layout--layout-test-dependencies-plugin');
+    $assert_session->elementExists('css', '.field--name-body');
+    $assert_session->linkExists('Save Layout');
+    $this->clickLink('Save Layout');
+    $this->drupalPostForm('admin/structure/menu/manage/myothermenu/delete', [], 'Delete');
+    $this->drupalGet('admin/structure/types/manage/bundle_with_section_field/display-layout/default');
+    $assert_session->elementNotExists('css', '.layout--layout-test-dependencies-plugin');
+    $assert_session->elementExists('css', '.field--name-body');
+
+    // Add a menu block.
+    $assert_session->linkExists('Add Block');
+    $this->clickLink('Add Block');
+    $assert_session->linkExists('My Menu');
+    $this->clickLink('My Menu');
+    $page->pressButton('Add Block');
+
+    // Add another block alongside the menu.
+    $assert_session->linkExists('Add Block');
+    $this->clickLink('Add Block');
+    $assert_session->linkExists('Powered by Drupal');
+    $this->clickLink('Powered by Drupal');
+    $page->pressButton('Add Block');
+
+    // Assert that the blocks are visible, and save the layout.
+    $assert_session->pageTextContains('Powered by Drupal');
+    $assert_session->pageTextContains('My Menu');
+    $assert_session->elementExists('css', '.block.menu--mymenu');
+    $assert_session->linkExists('Save Layout');
+    $this->clickLink('Save Layout');
+
+    // Delete the menu.
+    $this->drupalPostForm('admin/structure/menu/manage/mymenu/delete', [], 'Delete');
+
+    // Ensure that the menu block is gone, but that the other block remains.
+    $this->drupalGet('admin/structure/types/manage/bundle_with_section_field/display-layout/default');
+    $assert_session->pageTextContains('Powered by Drupal');
+    $assert_session->pageTextNotContains('My Menu');
+    $assert_session->elementNotExists('css', '.block.menu--mymenu');
+  }
+
+}

core/modules/layout_builder/tests/src/Functional/LayoutSectionTest.php

@@ -0,0 +1,379 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Functional;
+
+use Drupal\language\Entity\ConfigurableLanguage;
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the rendering of a layout section field.
+ *
+ * @group layout_builder
+ */
+class LayoutSectionTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['field_ui', 'layout_builder', 'node', 'block_test'];
+
+  /**
+   * The name of the layout section field.
+   *
+   * @var string
+   */
+  protected $fieldName = 'layout_builder__layout';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->createContentType([
+      'type' => 'bundle_without_section_field',
+    ]);
+    $this->createContentType([
+      'type' => 'bundle_with_section_field',
+    ]);
+
+    LayoutBuilderEntityViewDisplay::load('node.bundle_with_section_field.default')
+      ->setOverridable()
+      ->save();
+
+    $this->drupalLogin($this->drupalCreateUser([
+      'configure any layout',
+      'administer node display',
+      'administer node fields',
+      'administer content types',
+    ], 'foobar'));
+  }
+
+  /**
+   * Provides test data for ::testLayoutSectionFormatter().
+   */
+  public function providerTestLayoutSectionFormatter() {
+    $data = [];
+    $data['block_with_global_context'] = [
+      [
+        [
+          'section' => new Section('layout_onecol', [], [
+            'baz' => new SectionComponent('baz', 'content', [
+              'id' => 'test_context_aware',
+              'context_mapping' => [
+                'user' => '@user.current_user_context:current_user',
+              ],
+            ]),
+          ]),
+        ],
+      ],
+      [
+        '.layout--onecol',
+        '#test_context_aware--username',
+      ],
+      [
+        'foobar',
+      ],
+      'user',
+      'user:2',
+      'UNCACHEABLE',
+    ];
+    $data['block_with_entity_context'] = [
+      [
+        [
+          'section' => new Section('layout_onecol', [], [
+            'baz' => new SectionComponent('baz', 'content', [
+              'id' => 'field_block:node:bundle_with_section_field:body',
+              'context_mapping' => [
+                'entity' => 'layout_builder.entity',
+              ],
+            ]),
+          ]),
+        ],
+      ],
+      [
+        '.layout--onecol',
+        '.field--name-body',
+      ],
+      [
+        'Body',
+        'The node body',
+      ],
+      '',
+      '',
+      'MISS',
+    ];
+    $data['single_section_single_block'] = [
+      [
+        [
+          'section' => new Section('layout_onecol', [], [
+            'baz' => new SectionComponent('baz', 'content', [
+              'id' => 'system_powered_by_block',
+            ]),
+          ]),
+        ],
+      ],
+      '.layout--onecol',
+      'Powered by',
+      '',
+      '',
+      'MISS',
+    ];
+    $data['multiple_sections'] = [
+      [
+        [
+          'section' => new Section('layout_onecol', [], [
+            'baz' => new SectionComponent('baz', 'content', [
+              'id' => 'system_powered_by_block',
+            ]),
+          ]),
+        ],
+        [
+          'section' => new Section('layout_twocol', [], [
+            'foo' => new SectionComponent('foo', 'first', [
+              'id' => 'test_block_instantiation',
+              'display_message' => 'foo text',
+            ]),
+            'bar' => new SectionComponent('bar', 'second', [
+              'id' => 'test_block_instantiation',
+              'display_message' => 'bar text',
+            ]),
+          ]),
+        ],
+      ],
+      [
+        '.layout--onecol',
+        '.layout--twocol',
+      ],
+      [
+        'Powered by',
+        'foo text',
+        'bar text',
+      ],
+      'user.permissions',
+      '',
+      'MISS',
+    ];
+    return $data;
+  }
+
+  /**
+   * Tests layout_section formatter output.
+   *
+   * @dataProvider providerTestLayoutSectionFormatter
+   */
+  public function testLayoutSectionFormatter($layout_data, $expected_selector, $expected_content, $expected_cache_contexts, $expected_cache_tags, $expected_dynamic_cache) {
+    $node = $this->createSectionNode($layout_data);
+
+    $canonical_url = $node->toUrl('canonical');
+    $this->drupalGet($canonical_url);
+    $this->assertLayoutSection($expected_selector, $expected_content, $expected_cache_contexts, $expected_cache_tags, $expected_dynamic_cache);
+
+    $this->drupalGet($canonical_url->toString() . '/layout');
+    $this->assertLayoutSection($expected_selector, $expected_content, $expected_cache_contexts, $expected_cache_tags, 'UNCACHEABLE');
+  }
+
+  /**
+   * Tests the access checking of the section formatter.
+   */
+  public function testLayoutSectionFormatterAccess() {
+    $node = $this->createSectionNode([
+      [
+        'section' => new Section('layout_onecol', [], [
+          'baz' => new SectionComponent('baz', 'content', [
+            'id' => 'test_access',
+          ]),
+        ]),
+      ],
+    ]);
+
+    // Restrict access to the block.
+    $this->container->get('state')->set('test_block_access', FALSE);
+
+    $this->drupalGet($node->toUrl('canonical'));
+    $this->assertLayoutSection('.layout--onecol', NULL, '', '', 'UNCACHEABLE');
+    // Ensure the block was not rendered.
+    $this->assertSession()->pageTextNotContains('Hello test world');
+
+    // Grant access to the block, and ensure it was rendered.
+    $this->container->get('state')->set('test_block_access', TRUE);
+    $this->drupalGet($node->toUrl('canonical'));
+    $this->assertLayoutSection('.layout--onecol', 'Hello test world', '', '', 'UNCACHEABLE');
+  }
+
+  /**
+   * Tests the multilingual support of the section formatter.
+   */
+  public function testMultilingualLayoutSectionFormatter() {
+    $this->container->get('module_installer')->install(['content_translation']);
+    $this->rebuildContainer();
+
+    ConfigurableLanguage::createFromLangcode('es')->save();
+    $this->container->get('content_translation.manager')->setEnabled('node', 'bundle_with_section_field', TRUE);
+
+    $entity = $this->createSectionNode([
+      [
+        'section' => new Section('layout_onecol', [], [
+          'baz' => new SectionComponent('baz', 'content', [
+            'id' => 'system_powered_by_block',
+          ]),
+        ]),
+      ],
+    ]);
+    $entity->addTranslation('es', [
+      'title' => 'Translated node title',
+      $this->fieldName => [
+        [
+          'section' => new Section('layout_twocol', [], [
+            'foo' => new SectionComponent('foo', 'first', [
+              'id' => 'test_block_instantiation',
+              'display_message' => 'foo text',
+            ]),
+            'bar' => new SectionComponent('bar', 'second', [
+              'id' => 'test_block_instantiation',
+              'display_message' => 'bar text',
+            ]),
+          ]),
+        ],
+      ],
+    ]);
+    $entity->save();
+
+    $this->drupalGet($entity->toUrl('canonical'));
+    $this->assertLayoutSection('.layout--onecol', 'Powered by');
+    $this->drupalGet($entity->toUrl('canonical')->setOption('prefix', 'es/'));
+    $this->assertLayoutSection('.layout--twocol', ['foo text', 'bar text']);
+  }
+
+  /**
+   * Ensures that the entity title is displayed.
+   */
+  public function testLayoutPageTitle() {
+    $this->drupalPlaceBlock('page_title_block');
+    $node = $this->createSectionNode([]);
+
+    $this->drupalGet($node->toUrl('canonical')->toString() . '/layout');
+    $this->assertSession()->titleEquals('Edit layout for The node title | Drupal');
+    $this->assertEquals('Edit layout for The node title', $this->cssSelect('h1.page-title')[0]->getText());
+  }
+
+  /**
+   * Tests that no Layout link shows without a section field.
+   */
+  public function testLayoutUrlNoSectionField() {
+    $node = $this->createNode([
+      'type' => 'bundle_without_section_field',
+      'title' => 'The node title',
+      'body' => [
+        [
+          'value' => 'The node body',
+        ],
+      ],
+    ]);
+    $node->save();
+
+    $this->drupalGet($node->toUrl('canonical')->toString() . '/layout');
+    $this->assertSession()->statusCodeEquals(404);
+  }
+
+  /**
+   * Tests that deleting a field removes it from the layout.
+   */
+  public function testLayoutDeletingField() {
+    $assert_session = $this->assertSession();
+
+    $this->drupalGet('/admin/structure/types/manage/bundle_with_section_field/display-layout/default');
+    $assert_session->statusCodeEquals(200);
+    $assert_session->elementExists('css', '.field--name-body');
+
+    // Delete the field from both bundles.
+    $this->drupalGet('/admin/structure/types/manage/bundle_without_section_field/fields/node.bundle_without_section_field.body/delete');
+    $this->submitForm([], 'Delete');
+    $this->drupalGet('/admin/structure/types/manage/bundle_with_section_field/display-layout/default');
+    $assert_session->statusCodeEquals(200);
+    $assert_session->elementExists('css', '.field--name-body');
+
+    $this->drupalGet('/admin/structure/types/manage/bundle_with_section_field/fields/node.bundle_with_section_field.body/delete');
+    $this->submitForm([], 'Delete');
+    $this->drupalGet('/admin/structure/types/manage/bundle_with_section_field/display-layout/default');
+    $assert_session->statusCodeEquals(200);
+    $assert_session->elementNotExists('css', '.field--name-body');
+  }
+
+  /**
+   * Tests that deleting a bundle removes the layout.
+   */
+  public function testLayoutDeletingBundle() {
+    $assert_session = $this->assertSession();
+
+    $display = LayoutBuilderEntityViewDisplay::load('node.bundle_with_section_field.default');
+    $this->assertInstanceOf(LayoutBuilderEntityViewDisplay::class, $display);
+
+    $this->drupalPostForm('/admin/structure/types/manage/bundle_with_section_field/delete', [], 'Delete');
+    $assert_session->statusCodeEquals(200);
+
+    $display = LayoutBuilderEntityViewDisplay::load('node.bundle_with_section_field.default');
+    $this->assertNull($display);
+  }
+
+  /**
+   * Asserts the output of a layout section.
+   *
+   * @param string|array $expected_selector
+   *   A selector or list of CSS selectors to find.
+   * @param string|array $expected_content
+   *   A string or list of strings to find.
+   * @param string $expected_cache_contexts
+   *   A string of cache contexts to be found in the header.
+   * @param string $expected_cache_tags
+   *   A string of cache tags to be found in the header.
+   * @param string $expected_dynamic_cache
+   *   The expected dynamic cache header. Either 'HIT', 'MISS' or 'UNCACHEABLE'.
+   */
+  protected function assertLayoutSection($expected_selector, $expected_content, $expected_cache_contexts = '', $expected_cache_tags = '', $expected_dynamic_cache = 'MISS') {
+    $assert_session = $this->assertSession();
+    // Find the given selector.
+    foreach ((array) $expected_selector as $selector) {
+      $element = $this->cssSelect($selector);
+      $this->assertNotEmpty($element);
+    }
+
+    // Find the given content.
+    foreach ((array) $expected_content as $content) {
+      $assert_session->pageTextContains($content);
+    }
+    if ($expected_cache_contexts) {
+      $assert_session->responseHeaderContains('X-Drupal-Cache-Contexts', $expected_cache_contexts);
+    }
+    if ($expected_cache_tags) {
+      $assert_session->responseHeaderContains('X-Drupal-Cache-Tags', $expected_cache_tags);
+    }
+    $assert_session->responseHeaderEquals('X-Drupal-Dynamic-Cache', $expected_dynamic_cache);
+  }
+
+  /**
+   * Creates a node with a section field.
+   *
+   * @param array $section_values
+   *   An array of values for a section field.
+   *
+   * @return \Drupal\node\NodeInterface
+   *   The node object.
+   */
+  protected function createSectionNode(array $section_values) {
+    return $this->createNode([
+      'type' => 'bundle_with_section_field',
+      'title' => 'The node title',
+      'body' => [
+        [
+          'value' => 'The node body',
+        ],
+      ],
+      $this->fieldName => $section_values,
+    ]);
+  }
+
+}

core/modules/layout_builder/tests/src/Functional/Update/SectionDependenciesUpdatePathTest.php

@@ -0,0 +1,42 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Functional\Update;
+
+use Drupal\Core\Entity\Entity\EntityViewDisplay;
+use Drupal\FunctionalTests\Update\UpdatePathTestBase;
+
+/**
+ * Tests the upgrade path for Layout Builder section dependencies.
+ *
+ * @group layout_builder
+ */
+class SectionDependenciesUpdatePathTest extends UpdatePathTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setDatabaseDumpFiles() {
+    $this->databaseDumpFiles = [
+      __DIR__ . '/../../../../../system/tests/fixtures/update/drupal-8.4.0.bare.standard.php.gz',
+      __DIR__ . '/../../../fixtures/update/section-dependencies.php',
+    ];
+  }
+
+  /**
+   * Tests the upgrade path for Layout Builder section dependencies.
+   */
+  public function testRunUpdates() {
+    $data = EntityViewDisplay::load('node.article.teaser')->toArray();
+    $this->assertNotContains('system.menu.myothermenu', $data['dependencies']['config']);
+    $this->assertNotContains('layout_builder', $data['dependencies']['module']);
+    $this->assertNotContains('layout_test', $data['dependencies']['module']);
+
+    $this->runUpdates();
+
+    $data = EntityViewDisplay::load('node.article.teaser')->toArray();
+    $this->assertContains('system.menu.myothermenu', $data['dependencies']['config']);
+    $this->assertContains('layout_builder', $data['dependencies']['module']);
+    $this->assertContains('layout_test', $data['dependencies']['module']);
+  }
+
+}

core/modules/layout_builder/tests/src/FunctionalJavascript/FieldBlockTest.php

@@ -0,0 +1,122 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\FunctionalJavascript;
+
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Plugin\Block\FieldBlock
+ *
+ * @group field
+ */
+class FieldBlockTest extends JavascriptTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['block', 'datetime', 'layout_builder', 'user'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $field_storage = FieldStorageConfig::create([
+      'field_name' => 'field_date',
+      'entity_type' => 'user',
+      'type' => 'datetime',
+    ]);
+    $field_storage->save();
+    $field = FieldConfig::create([
+      'field_storage' => $field_storage,
+      'bundle' => 'user',
+      'label' => 'Date field',
+    ]);
+    $field->save();
+
+    $user = $this->drupalCreateUser([
+      'administer blocks',
+      'access administration pages',
+    ]);
+    $user->field_date = '1978-11-19T05:00:00';
+    $user->save();
+    $this->drupalLogin($user);
+  }
+
+  /**
+   * Tests configuring a field block for a user field.
+   */
+  public function testFieldBlock() {
+    $page = $this->getSession()->getPage();
+    $assert_session = $this->assertSession();
+
+    // Assert that the field value is not displayed.
+    $this->drupalGet('admin');
+    $assert_session->pageTextNotContains('Sunday, November 19, 1978 - 16:00');
+
+    $this->drupalGet('admin/structure/block');
+    $this->clickLink('Place block');
+    $assert_session->assertWaitOnAjaxRequest();
+
+    // Ensure that fields without any formatters are not available.
+    $assert_session->pageTextNotContains('Password');
+    // Ensure that non-display-configurable fields are not available.
+    $assert_session->pageTextNotContains('Initial email');
+
+    $assert_session->pageTextContains('Date field');
+    $block_url = 'admin/structure/block/add/field_block%3Auser%3Auser%3Afield_date/classy';
+    $assert_session->linkByHrefExists($block_url);
+
+    $this->drupalGet($block_url);
+    $page->fillField('region', 'content');
+
+    // Assert the default formatter configuration.
+    $assert_session->fieldValueEquals('settings[formatter][type]', 'datetime_default');
+    $assert_session->fieldValueEquals('settings[formatter][settings][format_type]', 'medium');
+
+    // Change the formatter.
+    $page->selectFieldOption('settings[formatter][type]', 'datetime_time_ago');
+    $assert_session->assertWaitOnAjaxRequest();
+    // Changing the formatter removes the old settings and introduces new ones.
+    $assert_session->fieldNotExists('settings[formatter][settings][format_type]');
+    $assert_session->fieldExists('settings[formatter][settings][granularity]');
+    $page->pressButton('Save block');
+    $assert_session->pageTextContains('The block configuration has been saved.');
+
+    // Configure the block and change the formatter again.
+    $this->clickLink('Configure');
+    $page->selectFieldOption('settings[formatter][type]', 'datetime_default');
+    $assert_session->assertWaitOnAjaxRequest();
+    $assert_session->fieldValueEquals('settings[formatter][settings][format_type]', 'medium');
+    $page->selectFieldOption('settings[formatter][settings][format_type]', 'long');
+
+    $page->pressButton('Save block');
+    $assert_session->pageTextContains('The block configuration has been saved.');
+
+    // Assert that the field value is updated.
+    $this->clickLink('Configure');
+    $assert_session->fieldValueEquals('settings[formatter][settings][format_type]', 'long');
+
+    // Assert that the field block is configured as expected.
+    $expected = [
+      'label' => 'above',
+      'type' => 'datetime_default',
+      'settings' => [
+        'format_type' => 'long',
+        'timezone_override' => '',
+      ],
+      'third_party_settings' => [],
+    ];
+    $config = $this->container->get('config.factory')->get('block.block.datefield');
+    $this->assertEquals($expected, $config->get('settings.formatter'));
+    $this->assertEquals(['field.field.user.user.field_date'], $config->get('dependencies.config'));
+
+    // Assert that the block is displaying the user field.
+    $this->drupalGet('admin');
+    $assert_session->pageTextContains('Sunday, November 19, 1978 - 16:00');
+  }
+
+}

core/modules/layout_builder/tests/src/Kernel/FieldBlockTest.php

@@ -0,0 +1,199 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Kernel;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\FormatterPluginManager;
+use Drupal\Core\Plugin\Context\ContextDefinition;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
+use Drupal\layout_builder\Plugin\Block\FieldBlock;
+use Prophecy\Prophecy\ProphecyInterface;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Plugin\Block\FieldBlock
+ * @group Field
+ */
+class FieldBlockTest extends EntityKernelTestBase {
+
+  /**
+   * Tests entity access.
+   *
+   * @covers ::blockAccess
+   * @dataProvider providerTestBlockAccessNotAllowed
+   */
+  public function testBlockAccessEntityNotAllowed($expected, $entity_access) {
+    $entity = $this->prophesize(FieldableEntityInterface::class);
+    $block = $this->getTestBlock($entity);
+
+    $account = $this->prophesize(AccountInterface::class);
+    $entity->access('view', $account->reveal(), TRUE)->willReturn($entity_access);
+    $entity->hasField()->shouldNotBeCalled();
+
+    $access = $block->access($account->reveal(), TRUE);
+    $this->assertSame($expected, $access->isAllowed());
+  }
+
+  /**
+   * Provides test data for ::testBlockAccessEntityNotAllowed().
+   */
+  public function providerTestBlockAccessNotAllowed() {
+    $data = [];
+    $data['entity_forbidden'] = [
+      FALSE,
+      AccessResult::forbidden(),
+    ];
+    $data['entity_neutral'] = [
+      FALSE,
+      AccessResult::neutral(),
+    ];
+    return $data;
+  }
+
+  /**
+   * Tests unfieldable entity.
+   *
+   * @covers ::blockAccess
+   */
+  public function testBlockAccessEntityAllowedNotFieldable() {
+    $entity = $this->prophesize(EntityInterface::class);
+    $block = $this->getTestBlock($entity);
+
+    $account = $this->prophesize(AccountInterface::class);
+    $entity->access('view', $account->reveal(), TRUE)->willReturn(AccessResult::allowed());
+
+    $access = $block->access($account->reveal(), TRUE);
+    $this->assertSame(FALSE, $access->isAllowed());
+  }
+
+  /**
+   * Tests fieldable entity without a particular field.
+   *
+   * @covers ::blockAccess
+   */
+  public function testBlockAccessEntityAllowedNoField() {
+    $entity = $this->prophesize(FieldableEntityInterface::class);
+    $block = $this->getTestBlock($entity);
+
+    $account = $this->prophesize(AccountInterface::class);
+    $entity->access('view', $account->reveal(), TRUE)->willReturn(AccessResult::allowed());
+    $entity->hasField('the_field_name')->willReturn(FALSE);
+    $entity->get('the_field_name')->shouldNotBeCalled();
+
+    $access = $block->access($account->reveal(), TRUE);
+    $this->assertSame(FALSE, $access->isAllowed());
+  }
+
+  /**
+   * Tests field access.
+   *
+   * @covers ::blockAccess
+   * @dataProvider providerTestBlockAccessNotAllowed
+   */
+  public function testBlockAccessEntityAllowedFieldNotAllowed($expected, $field_access) {
+    $entity = $this->prophesize(FieldableEntityInterface::class);
+    $block = $this->getTestBlock($entity);
+
+    $account = $this->prophesize(AccountInterface::class);
+    $entity->access('view', $account->reveal(), TRUE)->willReturn(AccessResult::allowed());
+    $entity->hasField('the_field_name')->willReturn(TRUE);
+    $field = $this->prophesize(FieldItemListInterface::class);
+    $entity->get('the_field_name')->willReturn($field->reveal());
+
+    $field->access('view', $account->reveal(), TRUE)->willReturn($field_access);
+    $field->isEmpty()->shouldNotBeCalled();
+
+    $access = $block->access($account->reveal(), TRUE);
+    $this->assertSame($expected, $access->isAllowed());
+  }
+
+  /**
+   * Tests populated vs empty build.
+   *
+   * @covers ::blockAccess
+   * @covers ::build
+   * @dataProvider providerTestBlockAccessEntityAllowedFieldHasValue
+   */
+  public function testBlockAccessEntityAllowedFieldHasValue($expected, $is_empty) {
+    $entity = $this->prophesize(FieldableEntityInterface::class);
+    $block = $this->getTestBlock($entity);
+
+    $account = $this->prophesize(AccountInterface::class);
+    $entity->access('view', $account->reveal(), TRUE)->willReturn(AccessResult::allowed());
+    $entity->hasField('the_field_name')->willReturn(TRUE);
+    $field = $this->prophesize(FieldItemListInterface::class);
+    $entity->get('the_field_name')->willReturn($field->reveal());
+
+    $field->access('view', $account->reveal(), TRUE)->willReturn(AccessResult::allowed());
+    $field->isEmpty()->willReturn($is_empty)->shouldBeCalled();
+
+    $access = $block->access($account->reveal(), TRUE);
+    $this->assertSame($expected, $access->isAllowed());
+  }
+
+  /**
+   * Provides test data for ::testBlockAccessEntityAllowedFieldHasValue().
+   */
+  public function providerTestBlockAccessEntityAllowedFieldHasValue() {
+    $data = [];
+    $data['empty'] = [
+      FALSE,
+      TRUE,
+    ];
+    $data['populated'] = [
+      TRUE,
+      FALSE,
+    ];
+    return $data;
+  }
+
+  /**
+   * Instantiates a block for testing.
+   *
+   * @param \Prophecy\Prophecy\ProphecyInterface $entity_prophecy
+   *   An entity prophecy for use as an entity context value.
+   * @param array $configuration
+   *   A configuration array containing information about the plugin instance.
+   * @param array $plugin_definition
+   *   The plugin implementation definition.
+   *
+   * @return \Drupal\layout_builder\Plugin\Block\FieldBlock
+   *   The block to test.
+   */
+  protected function getTestBlock(ProphecyInterface $entity_prophecy, array $configuration = [], array $plugin_definition = []) {
+    $entity_prophecy->getCacheContexts()->willReturn([]);
+    $entity_prophecy->getCacheTags()->willReturn([]);
+    $entity_prophecy->getCacheMaxAge()->willReturn(0);
+
+    $plugin_definition += [
+      'provider' => 'test',
+      'default_formatter' => '',
+      'category' => 'Test',
+      'admin_label' => 'Test Block',
+      'bundles' => ['entity_test'],
+      'context' => [
+        'entity' => new ContextDefinition('entity:entity_test', 'Test', TRUE),
+      ],
+    ];
+    $entity_field_manager = $this->prophesize(EntityFieldManagerInterface::class);
+    $formatter_manager = $this->prophesize(FormatterPluginManager::class);
+    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
+
+    $block = new FieldBlock(
+      $configuration,
+      'field_block:entity_test:entity_test:the_field_name',
+      $plugin_definition,
+      $entity_field_manager->reveal(),
+      $formatter_manager->reveal(),
+      $module_handler->reveal()
+    );
+    $block->setContextValue('entity', $entity_prophecy->reveal());
+    return $block;
+  }
+
+}

core/modules/layout_builder/tests/src/Kernel/LayoutBuilderCompatibilityTestBase.php

@@ -0,0 +1,122 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Kernel;
+
+use Drupal\Core\Entity\Entity\EntityViewDisplay;
+use Drupal\Core\Entity\EntityInterface;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
+
+/**
+ * Tests Layout Builder's compatibility with existing systems.
+ */
+abstract class LayoutBuilderCompatibilityTestBase extends EntityKernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'layout_discovery',
+  ];
+
+  /**
+   * The entity view display.
+   *
+   * @var \Drupal\field_layout\Display\EntityDisplayWithLayoutInterface
+   */
+  protected $display;
+
+  /**
+   * The entity being rendered.
+   *
+   * @var \Drupal\Core\Entity\FieldableEntityInterface
+   */
+  protected $entity;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->installEntitySchema('entity_test_base_field_display');
+    $this->installConfig(['filter']);
+    $this->installSchema('system', ['key_value_expire']);
+
+    // Set up a non-admin user that is allowed to view test entities.
+    \Drupal::currentUser()->setAccount($this->createUser(['uid' => 2], ['view test entity']));
+
+    \Drupal::service('theme_handler')->install(['classy']);
+    $this->config('system.theme')->set('default', 'classy')->save();
+
+    $field_storage = FieldStorageConfig::create([
+      'entity_type' => 'entity_test_base_field_display',
+      'field_name' => 'test_field_display_configurable',
+      'type' => 'boolean',
+    ]);
+    $field_storage->save();
+    FieldConfig::create([
+      'field_storage' => $field_storage,
+      'bundle' => 'entity_test_base_field_display',
+      'label' => 'FieldConfig with configurable display',
+    ])->save();
+
+    $this->display = EntityViewDisplay::create([
+      'targetEntityType' => 'entity_test_base_field_display',
+      'bundle' => 'entity_test_base_field_display',
+      'mode' => 'default',
+      'status' => TRUE,
+    ]);
+    $this->display
+      ->setComponent('test_field_display_configurable', ['weight' => 5])
+      ->save();
+
+    // Create an entity with fields that are configurable and non-configurable.
+    $entity_storage = $this->container->get('entity_type.manager')->getStorage('entity_test_base_field_display');
+    // @todo Remove langcode workarounds after resolving
+    //   https://www.drupal.org/node/2915034.
+    $this->entity = $entity_storage->createWithSampleValues('entity_test_base_field_display', [
+      'langcode' => 'en',
+      'langcode_default' => TRUE,
+    ]);
+    $this->entity->save();
+  }
+
+  /**
+   * Installs the Layout Builder.
+   *
+   * Also configures and reloads the entity display, and reloads the entity.
+   */
+  protected function installLayoutBuilder() {
+    $this->container->get('module_installer')->install(['layout_builder']);
+    $this->refreshServices();
+
+    $this->display = $this->reloadEntity($this->display);
+    $this->display->setOverridable()->save();
+    $this->entity = $this->reloadEntity($this->entity);
+  }
+
+  /**
+   * Asserts that the rendered entity has the correct fields.
+   *
+   * @param \Drupal\Core\Entity\EntityInterface $entity
+   *   The entity to render.
+   * @param array $attributes
+   *   An array of field attributes to assert.
+   *
+   * @return string
+   *   The rendered string output (typically HTML).
+   */
+  protected function assertFieldAttributes(EntityInterface $entity, array $attributes) {
+    $view_builder = $this->container->get('entity_type.manager')->getViewBuilder($entity->getEntityTypeId());
+    $build = $view_builder->view($entity);
+    $this->render($build);
+
+    $actual = array_map(function (\SimpleXMLElement $element) {
+      return (string) $element->attributes();
+    }, $this->cssSelect('.field'));
+    $this->assertSame($attributes, $actual);
+  }
+
+}

core/modules/layout_builder/tests/src/Kernel/LayoutBuilderEntityViewDisplayTest.php

@@ -0,0 +1,43 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Kernel;
+
+use Drupal\Core\Config\Schema\SchemaIncompleteException;
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay
+ *
+ * @group layout_builder
+ */
+class LayoutBuilderEntityViewDisplayTest extends SectionStorageTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getSectionStorage(array $section_data) {
+    $display = LayoutBuilderEntityViewDisplay::create([
+      'targetEntityType' => 'entity_test',
+      'bundle' => 'entity_test',
+      'mode' => 'default',
+      'status' => TRUE,
+      'third_party_settings' => [
+        'layout_builder' => [
+          'sections' => $section_data,
+        ],
+      ],
+    ]);
+    $display->save();
+    return $display;
+  }
+
+  /**
+   * Tests that configuration schema enforces valid values.
+   */
+  public function testInvalidConfiguration() {
+    $this->setExpectedException(SchemaIncompleteException::class);
+    $this->sectionStorage->getSection(0)->getComponent('first-uuid')->setConfiguration(['id' => 'foo', 'bar' => 'baz']);
+    $this->sectionStorage->save();
+  }
+
+}

core/modules/layout_builder/tests/src/Kernel/LayoutBuilderFieldLayoutCompatibilityTest.php

@@ -0,0 +1,79 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Kernel;
+
+use Drupal\layout_builder\Section;
+
+/**
+ * Ensures that Layout Builder and Field Layout are compatible with each other.
+ *
+ * @group layout_builder
+ */
+class LayoutBuilderFieldLayoutCompatibilityTest extends LayoutBuilderCompatibilityTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'field_layout',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->display
+      ->setLayoutId('layout_twocol')
+      ->save();
+  }
+
+  /**
+   * Tests the compatibility of Layout Builder and Field Layout.
+   */
+  public function testCompatibility() {
+    // Ensure that the configurable field is shown in the correct region and
+    // that the non-configurable field is shown outside the layout.
+    $expected_fields = [
+      'field field--name-name field--type-string field--label-hidden field__item',
+      'field field--name-test-field-display-configurable field--type-boolean field--label-above',
+      'clearfix text-formatted field field--name-test-display-configurable field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-display-non-configurable field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-display-multiple field--type-text field--label-above',
+    ];
+    $this->assertFieldAttributes($this->entity, $expected_fields);
+    $this->assertNotEmpty($this->cssSelect('.layout__region--first .field--name-test-display-configurable'));
+    $this->assertNotEmpty($this->cssSelect('.layout__region--first .field--name-test-field-display-configurable'));
+    $this->assertNotEmpty($this->cssSelect('.field--name-test-display-non-configurable'));
+    $this->assertEmpty($this->cssSelect('.layout__region .field--name-test-display-non-configurable'));
+
+    $this->installLayoutBuilder();
+
+    // Without using Layout Builder for an override, the result has not changed.
+    $this->assertFieldAttributes($this->entity, $expected_fields);
+
+    // Add a layout override.
+    /** @var \Drupal\layout_builder\SectionStorageInterface $field_list */
+    $field_list = $this->entity->get('layout_builder__layout');
+    $field_list->appendSection(new Section('layout_onecol'));
+    $this->entity->save();
+
+    // The rendered entity has now changed. The non-configurable field is shown
+    // outside the layout, the configurable field is not shown at all, and the
+    // layout itself is rendered (but empty).
+    $new_expected_fields = [
+      'field field--name-name field--type-string field--label-hidden field__item',
+      'clearfix text-formatted field field--name-test-display-non-configurable field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-display-multiple field--type-text field--label-above',
+    ];
+    $this->assertFieldAttributes($this->entity, $new_expected_fields);
+    $this->assertNotEmpty($this->cssSelect('.layout--onecol'));
+
+    // Removing the layout restores the original rendering of the entity.
+    $field_list->removeSection(0);
+    $this->entity->save();
+    $this->assertFieldAttributes($this->entity, $expected_fields);
+  }
+
+}

core/modules/layout_builder/tests/src/Kernel/LayoutBuilderInstallTest.php

@@ -0,0 +1,90 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Kernel;
+
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+use Drupal\layout_builder\Section;
+
+/**
+ * Ensures that Layout Builder and core EntityViewDisplays are compatible.
+ *
+ * @group layout_builder
+ */
+class LayoutBuilderInstallTest extends LayoutBuilderCompatibilityTestBase {
+
+  /**
+   * Tests the compatibility of Layout Builder with existing entity displays.
+   */
+  public function testCompatibility() {
+    // Ensure that the fields are shown.
+    $expected_fields = [
+      'field field--name-name field--type-string field--label-hidden field__item',
+      'field field--name-test-field-display-configurable field--type-boolean field--label-above',
+      'clearfix text-formatted field field--name-test-display-configurable field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-display-non-configurable field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-display-multiple field--type-text field--label-above',
+    ];
+    $this->assertFieldAttributes($this->entity, $expected_fields);
+
+    $this->installLayoutBuilder();
+
+    // Without using Layout Builder for an override, the result has not changed.
+    $this->assertFieldAttributes($this->entity, $expected_fields);
+
+    // Add a layout override.
+    $this->entity->get('layout_builder__layout')->appendSection(new Section('layout_onecol'));
+    $this->entity->save();
+
+    // The rendered entity has now changed. The non-configurable field is shown
+    // outside the layout, the configurable field is not shown at all, and the
+    // layout itself is rendered (but empty).
+    $new_expected_fields = [
+      'field field--name-name field--type-string field--label-hidden field__item',
+      'clearfix text-formatted field field--name-test-display-non-configurable field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-display-multiple field--type-text field--label-above',
+    ];
+    $this->assertFieldAttributes($this->entity, $new_expected_fields);
+    $this->assertNotEmpty($this->cssSelect('.layout--onecol'));
+
+    // Removing the layout restores the original rendering of the entity.
+    $this->entity->get('layout_builder__layout')->removeSection(0);
+    $this->entity->save();
+    $this->assertFieldAttributes($this->entity, $expected_fields);
+
+    // Test that adding a new field after Layout Builder has been installed will
+    // add the new field to the default region of the first section.
+    $field_storage = FieldStorageConfig::create([
+      'entity_type' => 'entity_test_base_field_display',
+      'field_name' => 'test_field_display_post_install',
+      'type' => 'text',
+    ]);
+    $field_storage->save();
+    FieldConfig::create([
+      'field_storage' => $field_storage,
+      'bundle' => 'entity_test_base_field_display',
+      'label' => 'FieldConfig with configurable display',
+    ])->save();
+
+    $this->entity = $this->reloadEntity($this->entity);
+    $this->entity->test_field_display_post_install = 'Test string';
+    $this->entity->save();
+
+    $this->display = $this->reloadEntity($this->display);
+    $this->display
+      ->setComponent('test_field_display_post_install', ['weight' => 50])
+      ->save();
+    $new_expected_fields = [
+      'field field--name-name field--type-string field--label-hidden field__item',
+      'field field--name-test-field-display-configurable field--type-boolean field--label-above',
+      'clearfix text-formatted field field--name-test-display-configurable field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-field-display-post-install field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-display-non-configurable field--type-text field--label-above',
+      'clearfix text-formatted field field--name-test-display-multiple field--type-text field--label-above',
+    ];
+    $this->assertFieldAttributes($this->entity, $new_expected_fields);
+    $this->assertNotEmpty($this->cssSelect('.layout--onecol'));
+    $this->assertText('Test string');
+  }
+
+}

core/modules/layout_builder/tests/src/Kernel/LayoutSectionItemListTest.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Kernel;
+
+use Drupal\entity_test\Entity\EntityTestBaseFieldDisplay;
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay;
+
+/**
+ * Tests the field type for Layout Sections.
+ *
+ * @coversDefaultClass \Drupal\layout_builder\Field\LayoutSectionItemList
+ *
+ * @group layout_builder
+ */
+class LayoutSectionItemListTest extends SectionStorageTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'field',
+    'text',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getSectionStorage(array $section_data) {
+    $this->installEntitySchema('entity_test_base_field_display');
+    LayoutBuilderEntityViewDisplay::create([
+      'targetEntityType' => 'entity_test_base_field_display',
+      'bundle' => 'entity_test_base_field_display',
+      'mode' => 'default',
+      'status' => TRUE,
+    ])->setOverridable()->save();
+
+    array_map(function ($row) {
+      return ['section' => $row];
+    }, $section_data);
+    $entity = EntityTestBaseFieldDisplay::create([
+      'name' => 'The test entity',
+      'layout_builder__layout' => $section_data,
+    ]);
+    $entity->save();
+    return $entity->get('layout_builder__layout');
+  }
+
+}

core/modules/layout_builder/tests/src/Kernel/SectionStorageTestBase.php

@@ -0,0 +1,152 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Kernel;
+
+use Drupal\KernelTests\Core\Entity\EntityKernelTestBase;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionComponent;
+
+/**
+ * Provides a base class for testing implementations of section storage.
+ */
+abstract class SectionStorageTestBase extends EntityKernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'layout_builder',
+    'layout_discovery',
+    'layout_test',
+  ];
+
+  /**
+   * The section storage implementation.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->installSchema('system', ['key_value_expire']);
+
+    $section_data = [
+      new Section('layout_test_plugin', [], [
+        'first-uuid' => new SectionComponent('first-uuid', 'content', ['id' => 'foo']),
+      ]),
+      new Section('layout_test_plugin', ['setting_1' => 'bar'], [
+        'second-uuid' => new SectionComponent('second-uuid', 'content', ['id' => 'foo']),
+      ]),
+    ];
+    $this->sectionStorage = $this->getSectionStorage($section_data);
+  }
+
+  /**
+   * Sets up the section storage entity.
+   *
+   * @param array $section_data
+   *   An array of section data.
+   *
+   * @return \Drupal\Core\Entity\EntityInterface
+   *   The entity.
+   */
+  abstract protected function getSectionStorage(array $section_data);
+
+  /**
+   * @covers ::getSections
+   */
+  public function testGetSections() {
+    $expected = [
+      new Section('layout_test_plugin', [], [
+        'first-uuid' => new SectionComponent('first-uuid', 'content', ['id' => 'foo']),
+      ]),
+      new Section('layout_test_plugin', ['setting_1' => 'bar'], [
+        'second-uuid' => new SectionComponent('second-uuid', 'content', ['id' => 'foo']),
+      ]),
+    ];
+    $this->assertSections($expected);
+  }
+
+  /**
+   * @covers ::getSection
+   */
+  public function testGetSection() {
+    $this->assertInstanceOf(Section::class, $this->sectionStorage->getSection(0));
+  }
+
+  /**
+   * @covers ::getSection
+   */
+  public function testGetSectionInvalidDelta() {
+    $this->setExpectedException(\OutOfBoundsException::class, 'Invalid delta "2"');
+    $this->sectionStorage->getSection(2);
+  }
+
+  /**
+   * @covers ::insertSection
+   */
+  public function testInsertSection() {
+    $expected = [
+      new Section('layout_test_plugin', [], [
+        'first-uuid' => new SectionComponent('first-uuid', 'content', ['id' => 'foo']),
+      ]),
+      new Section('setting_1'),
+      new Section('layout_test_plugin', ['setting_1' => 'bar'], [
+        'second-uuid' => new SectionComponent('second-uuid', 'content', ['id' => 'foo']),
+      ]),
+    ];
+
+    $this->sectionStorage->insertSection(1, new Section('setting_1'));
+    $this->assertSections($expected);
+  }
+
+  /**
+   * @covers ::appendSection
+   */
+  public function testAppendSection() {
+    $expected = [
+      new Section('layout_test_plugin', [], [
+        'first-uuid' => new SectionComponent('first-uuid', 'content', ['id' => 'foo']),
+      ]),
+      new Section('layout_test_plugin', ['setting_1' => 'bar'], [
+        'second-uuid' => new SectionComponent('second-uuid', 'content', ['id' => 'foo']),
+      ]),
+      new Section('foo'),
+    ];
+
+    $this->sectionStorage->appendSection(new Section('foo'));
+    $this->assertSections($expected);
+  }
+
+  /**
+   * @covers ::removeSection
+   */
+  public function testRemoveSection() {
+    $expected = [
+      new Section('layout_test_plugin', ['setting_1' => 'bar'], [
+        'second-uuid' => new SectionComponent('second-uuid', 'content', ['id' => 'foo']),
+      ]),
+    ];
+
+    $this->sectionStorage->removeSection(0);
+    $this->assertSections($expected);
+  }
+
+  /**
+   * Asserts that the field list has the expected sections.
+   *
+   * @param \Drupal\layout_builder\Section[] $expected
+   *   The expected sections.
+   */
+  protected function assertSections(array $expected) {
+    $result = $this->sectionStorage->getSections();
+    $this->assertEquals($expected, $result);
+    $this->assertSame(array_keys($expected), array_keys($result));
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/BlockComponentRenderArrayTest.php

@@ -0,0 +1,222 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent;
+use Drupal\layout_builder\EventSubscriber\BlockComponentRenderArray;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\EventSubscriber\BlockComponentRenderArray
+ * @group layout_builder
+ */
+class BlockComponentRenderArrayTest extends UnitTestCase {
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $account;
+
+  /**
+   * The block plugin manager.
+   *
+   * @var \Drupal\Core\Block\BlockManagerInterface
+   */
+  protected $blockManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->blockManager = $this->prophesize(BlockManagerInterface::class);
+    $this->account = $this->prophesize(AccountInterface::class);
+
+    $container = new ContainerBuilder();
+    $container->set('plugin.manager.block', $this->blockManager->reveal());
+    \Drupal::setContainer($container);
+  }
+
+  /**
+   * @covers ::onBuildRender
+   */
+  public function testOnBuildRender() {
+    $block = $this->prophesize(BlockPluginInterface::class);
+    $access_result = AccessResult::allowed();
+    $block->access($this->account->reveal(), TRUE)->willReturn($access_result)->shouldBeCalled();
+    $block->getCacheContexts()->willReturn([]);
+    $block->getCacheTags()->willReturn(['test']);
+    $block->getCacheMaxAge()->willReturn(Cache::PERMANENT);
+    $block->getConfiguration()->willReturn([]);
+    $block->getPluginId()->willReturn('block_plugin_id');
+    $block->getBaseId()->willReturn('block_plugin_id');
+    $block->getDerivativeId()->willReturn(NULL);
+
+    $block_content = ['#markup' => 'The block content.'];
+    $block->build()->willReturn($block_content);
+    $this->blockManager->createInstance('some_block_id', ['id' => 'some_block_id'])->willReturn($block->reveal());
+
+    $component = new SectionComponent('some-uuid', 'some-region', ['id' => 'some_block_id']);
+    $contexts = [];
+    $in_preview = FALSE;
+    $event = new SectionComponentBuildRenderArrayEvent($component, $contexts, $in_preview);
+
+    $subscriber = new BlockComponentRenderArray($this->account->reveal());
+
+    $expected_build = [
+      '#theme' => 'block',
+      '#weight' => 0,
+      '#configuration' => [],
+      '#plugin_id' => 'block_plugin_id',
+      '#base_plugin_id' => 'block_plugin_id',
+      '#derivative_plugin_id' => NULL,
+      'content' => $block_content,
+    ];
+
+    $expected_cache = $expected_build + [
+      '#cache' => [
+        'contexts' => [],
+        'tags' => ['test'],
+        'max-age' => -1,
+      ],
+    ];
+
+    $subscriber->onBuildRender($event);
+    $result = $event->getBuild();
+    $this->assertEquals($expected_build, $result);
+    $event->getCacheableMetadata()->applyTo($result);
+    $this->assertEquals($expected_cache, $result);
+  }
+
+  /**
+   * @covers ::onBuildRender
+   */
+  public function testOnBuildRenderDenied() {
+    $block = $this->prophesize(BlockPluginInterface::class);
+    $access_result = AccessResult::forbidden();
+    $block->access($this->account->reveal(), TRUE)->willReturn($access_result)->shouldBeCalled();
+    $block->getCacheContexts()->shouldNotBeCalled();
+    $block->getCacheTags()->shouldNotBeCalled();
+    $block->getCacheMaxAge()->shouldNotBeCalled();
+    $block->getConfiguration()->shouldNotBeCalled();
+    $block->getPluginId()->shouldNotBeCalled();
+    $block->getBaseId()->shouldNotBeCalled();
+    $block->getDerivativeId()->shouldNotBeCalled();
+
+    $block_content = ['#markup' => 'The block content.'];
+    $block->build()->willReturn($block_content);
+    $this->blockManager->createInstance('some_block_id', ['id' => 'some_block_id'])->willReturn($block->reveal());
+
+    $component = new SectionComponent('some-uuid', 'some-region', ['id' => 'some_block_id']);
+    $contexts = [];
+    $in_preview = FALSE;
+    $event = new SectionComponentBuildRenderArrayEvent($component, $contexts, $in_preview);
+
+    $subscriber = new BlockComponentRenderArray($this->account->reveal());
+
+    $expected_build = [];
+    $expected_cache = [
+      '#cache' => [
+        'contexts' => [],
+        'tags' => [],
+        'max-age' => -1,
+      ],
+    ];
+
+    $subscriber->onBuildRender($event);
+    $result = $event->getBuild();
+    $this->assertEquals($expected_build, $result);
+    $event->getCacheableMetadata()->applyTo($result);
+    $this->assertEquals($expected_cache, $result);
+  }
+
+  /**
+   * @covers ::onBuildRender
+   */
+  public function testOnBuildRenderInPreview() {
+    $block = $this->prophesize(BlockPluginInterface::class);
+    $block->access($this->account->reveal(), TRUE)->shouldNotBeCalled();
+    $block->getCacheContexts()->willReturn([]);
+    $block->getCacheTags()->willReturn(['test']);
+    $block->getCacheMaxAge()->willReturn(Cache::PERMANENT);
+    $block->getConfiguration()->willReturn([]);
+    $block->getPluginId()->willReturn('block_plugin_id');
+    $block->getBaseId()->willReturn('block_plugin_id');
+    $block->getDerivativeId()->willReturn(NULL);
+
+    $block_content = ['#markup' => 'The block content.'];
+    $block->build()->willReturn($block_content);
+    $this->blockManager->createInstance('some_block_id', ['id' => 'some_block_id'])->willReturn($block->reveal());
+
+    $component = new SectionComponent('some-uuid', 'some-region', ['id' => 'some_block_id']);
+    $contexts = [];
+    $in_preview = TRUE;
+    $event = new SectionComponentBuildRenderArrayEvent($component, $contexts, $in_preview);
+
+    $subscriber = new BlockComponentRenderArray($this->account->reveal());
+
+    $expected_build = [
+      '#theme' => 'block',
+      '#weight' => 0,
+      '#configuration' => [],
+      '#plugin_id' => 'block_plugin_id',
+      '#base_plugin_id' => 'block_plugin_id',
+      '#derivative_plugin_id' => NULL,
+      'content' => $block_content,
+    ];
+
+    $expected_cache = $expected_build + [
+      '#cache' => [
+        'contexts' => [],
+        'tags' => ['test'],
+        'max-age' => 0,
+      ],
+    ];
+
+    $subscriber->onBuildRender($event);
+    $result = $event->getBuild();
+    $this->assertEquals($expected_build, $result);
+    $event->getCacheableMetadata()->applyTo($result);
+    $this->assertEquals($expected_cache, $result);
+  }
+
+  /**
+   * @covers ::onBuildRender
+   */
+  public function testOnBuildRenderNoBlock() {
+    $this->blockManager->createInstance('some_block_id', ['id' => 'some_block_id'])->willReturn(NULL);
+
+    $component = new SectionComponent('some-uuid', 'some-region', ['id' => 'some_block_id']);
+    $contexts = [];
+    $in_preview = FALSE;
+    $event = new SectionComponentBuildRenderArrayEvent($component, $contexts, $in_preview);
+
+    $subscriber = new BlockComponentRenderArray($this->account->reveal());
+
+    $expected_build = [];
+    $expected_cache = [
+      '#cache' => [
+        'contexts' => [],
+        'tags' => [],
+        'max-age' => -1,
+      ],
+    ];
+
+    $subscriber->onBuildRender($event);
+    $result = $event->getBuild();
+    $this->assertEquals($expected_build, $result);
+    $event->getCacheableMetadata()->applyTo($result);
+    $this->assertEquals($expected_cache, $result);
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/DefaultsSectionStorageTest.php

@@ -0,0 +1,356 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityType;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\layout_builder\Entity\LayoutBuilderSampleEntityGenerator;
+use Drupal\layout_builder\Plugin\SectionStorage\DefaultsSectionStorage;
+use Drupal\layout_builder\SectionStorage\SectionStorageDefinition;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Component\Routing\Route;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Plugin\SectionStorage\DefaultsSectionStorage
+ *
+ * @group layout_builder
+ */
+class DefaultsSectionStorageTest extends UnitTestCase {
+
+  /**
+   * The plugin.
+   *
+   * @var \Drupal\layout_builder\Plugin\SectionStorage\DefaultsSectionStorage
+   */
+  protected $plugin;
+
+  /**
+   * The entity manager.
+   *
+   * @var \Drupal\Core\Entity\EntityManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->entityTypeManager = $this->prophesize(EntityTypeManagerInterface::class);
+    $entity_type_bundle_info = $this->prophesize(EntityTypeBundleInfoInterface::class);
+    $sample_entity_generator = $this->prophesize(LayoutBuilderSampleEntityGenerator::class);
+
+    $definition = new SectionStorageDefinition([
+      'id' => 'defaults',
+      'class' => DefaultsSectionStorage::class,
+    ]);
+    $this->plugin = new DefaultsSectionStorage([], '', $definition, $this->entityTypeManager->reveal(), $entity_type_bundle_info->reveal(), $sample_entity_generator->reveal());
+  }
+
+  /**
+   * @covers ::extractIdFromRoute
+   *
+   * @dataProvider providerTestExtractIdFromRoute
+   */
+  public function testExtractIdFromRoute($expected, $value, array $defaults) {
+    $result = $this->plugin->extractIdFromRoute($value, [], 'the_parameter_name', $defaults);
+    $this->assertSame($expected, $result);
+  }
+
+  /**
+   * Provides data for ::testExtractIdFromRoute().
+   */
+  public function providerTestExtractIdFromRoute() {
+    $data = [];
+    $data['with value'] = [
+      'foo.bar.baz',
+      'foo.bar.baz',
+      [],
+    ];
+    $data['empty value, without bundle'] = [
+      'my_entity_type.bundle_name.default',
+      '',
+      [
+        'entity_type_id' => 'my_entity_type',
+        'view_mode_name' => 'default',
+        'bundle_key' => 'my_bundle',
+        'my_bundle' => 'bundle_name',
+      ],
+    ];
+    $data['empty value, with bundle'] = [
+      'my_entity_type.bundle_name.default',
+      '',
+      [
+        'entity_type_id' => 'my_entity_type',
+        'view_mode_name' => 'default',
+        'bundle' => 'bundle_name',
+      ],
+    ];
+    $data['without value, empty defaults'] = [
+      NULL,
+      '',
+      [],
+    ];
+    return $data;
+  }
+
+  /**
+   * @covers ::getSectionListFromId
+   *
+   * @dataProvider providerTestGetSectionListFromId
+   */
+  public function testGetSectionListFromId($success, $expected_entity_id, $value) {
+    if ($expected_entity_id) {
+      $entity_storage = $this->prophesize(EntityStorageInterface::class);
+      $entity_storage->load($expected_entity_id)->willReturn('the_return_value');
+
+      $this->entityTypeManager->getDefinition('entity_view_display')->willReturn(new EntityType(['id' => 'entity_view_display']));
+      $this->entityTypeManager->getStorage('entity_view_display')->willReturn($entity_storage->reveal());
+    }
+    else {
+      $this->entityTypeManager->getDefinition('entity_view_display')->shouldNotBeCalled();
+      $this->entityTypeManager->getStorage('entity_view_display')->shouldNotBeCalled();
+    }
+
+    if (!$success) {
+      $this->setExpectedException(\InvalidArgumentException::class);
+    }
+
+    $result = $this->plugin->getSectionListFromId($value);
+    if ($success) {
+      $this->assertEquals('the_return_value', $result);
+    }
+  }
+
+  /**
+   * Provides data for ::testGetSectionListFromId().
+   */
+  public function providerTestGetSectionListFromId() {
+    $data = [];
+    $data['with value'] = [
+      TRUE,
+      'foo.bar.baz',
+      'foo.bar.baz',
+    ];
+    $data['without value, empty defaults'] = [
+      FALSE,
+      NULL,
+      '',
+    ];
+    return $data;
+  }
+
+  /**
+   * @covers ::getSectionListFromId
+   */
+  public function testGetSectionListFromIdCreate() {
+    $expected = 'the_return_value';
+    $value = 'foo.bar.baz';
+    $expected_create_values = [
+      'targetEntityType' => 'foo',
+      'bundle' => 'bar',
+      'mode' => 'baz',
+      'status' => TRUE,
+    ];
+    $entity_storage = $this->prophesize(EntityStorageInterface::class);
+    $entity_storage->load($value)->willReturn(NULL);
+    $entity_storage->create($expected_create_values)->willReturn($expected);
+
+    $this->entityTypeManager->getDefinition('entity_view_display')->willReturn(new EntityType(['id' => 'entity_view_display']));
+    $this->entityTypeManager->getStorage('entity_view_display')->willReturn($entity_storage->reveal());
+
+    $result = $this->plugin->getSectionListFromId($value);
+    $this->assertSame($expected, $result);
+  }
+
+  /**
+   * @covers ::buildRoutes
+   * @covers ::getEntityTypes
+   */
+  public function testBuildRoutes() {
+    $entity_types = [];
+
+    $not_fieldable = $this->prophesize(EntityTypeInterface::class);
+    $not_fieldable->entityClassImplements(FieldableEntityInterface::class)->willReturn(FALSE);
+    $entity_types['not_fieldable'] = $not_fieldable->reveal();
+
+    $no_view_builder = $this->prophesize(EntityTypeInterface::class);
+    $no_view_builder->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $no_view_builder->hasViewBuilderClass()->willReturn(FALSE);
+    $entity_types['no_view_builder'] = $no_view_builder->reveal();
+
+    $no_field_ui_route = $this->prophesize(EntityTypeInterface::class);
+    $no_field_ui_route->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $no_field_ui_route->hasViewBuilderClass()->willReturn(TRUE);
+    $no_field_ui_route->get('field_ui_base_route')->willReturn(NULL);
+    $entity_types['no_field_ui_route'] = $no_field_ui_route->reveal();
+
+    $unknown_field_ui_route = $this->prophesize(EntityTypeInterface::class);
+    $unknown_field_ui_route->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $unknown_field_ui_route->hasViewBuilderClass()->willReturn(TRUE);
+    $unknown_field_ui_route->get('field_ui_base_route')->willReturn('unknown');
+    $entity_types['unknown_field_ui_route'] = $unknown_field_ui_route->reveal();
+
+    $with_bundle_key = $this->prophesize(EntityTypeInterface::class);
+    $with_bundle_key->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $with_bundle_key->hasViewBuilderClass()->willReturn(TRUE);
+    $with_bundle_key->get('field_ui_base_route')->willReturn('known');
+    $with_bundle_key->hasKey('bundle')->willReturn(TRUE);
+    $with_bundle_key->getBundleEntityType()->willReturn('my_bundle_type');
+    $entity_types['with_bundle_key'] = $with_bundle_key->reveal();
+
+    $with_bundle_parameter = $this->prophesize(EntityTypeInterface::class);
+    $with_bundle_parameter->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $with_bundle_parameter->hasViewBuilderClass()->willReturn(TRUE);
+    $with_bundle_parameter->get('field_ui_base_route')->willReturn('with_bundle');
+    $entity_types['with_bundle_parameter'] = $with_bundle_parameter->reveal();
+    $this->entityTypeManager->getDefinitions()->willReturn($entity_types);
+
+    $expected = [
+      'known' => new Route('/admin/entity/whatever', [], [], ['_admin_route' => TRUE]),
+      'with_bundle' => new Route('/admin/entity/{bundle}'),
+      'layout_builder.defaults.with_bundle_key.view' => new Route(
+        '/admin/entity/whatever/display-layout/{view_mode_name}',
+        [
+          'entity_type_id' => 'with_bundle_key',
+          'bundle_key' => 'my_bundle_type',
+          'section_storage_type' => 'defaults',
+          'section_storage' => '',
+          'is_rebuilding' => FALSE,
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::layout',
+          '_title_callback' => '\Drupal\layout_builder\Controller\LayoutBuilderController::title',
+        ],
+        [
+          '_field_ui_view_mode_access' => 'administer with_bundle_key display',
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+          ],
+          '_layout_builder' => TRUE,
+          '_admin_route' => FALSE,
+        ]
+      ),
+      'layout_builder.defaults.with_bundle_key.save' => new Route(
+        '/admin/entity/whatever/display-layout/{view_mode_name}/save',
+        [
+          'entity_type_id' => 'with_bundle_key',
+          'bundle_key' => 'my_bundle_type',
+          'section_storage_type' => 'defaults',
+          'section_storage' => '',
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::saveLayout',
+        ],
+        [
+          '_field_ui_view_mode_access' => 'administer with_bundle_key display',
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+          ],
+          '_layout_builder' => TRUE,
+          '_admin_route' => FALSE,
+        ]
+      ),
+      'layout_builder.defaults.with_bundle_key.cancel' => new Route(
+        '/admin/entity/whatever/display-layout/{view_mode_name}/cancel',
+        [
+          'entity_type_id' => 'with_bundle_key',
+          'bundle_key' => 'my_bundle_type',
+          'section_storage_type' => 'defaults',
+          'section_storage' => '',
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::cancelLayout',
+        ],
+        [
+          '_field_ui_view_mode_access' => 'administer with_bundle_key display',
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+          ],
+          '_layout_builder' => TRUE,
+          '_admin_route' => FALSE,
+        ]
+      ),
+      'layout_builder.defaults.with_bundle_parameter.view' => new Route(
+        '/admin/entity/{bundle}/display-layout/{view_mode_name}',
+        [
+          'entity_type_id' => 'with_bundle_parameter',
+          'section_storage_type' => 'defaults',
+          'section_storage' => '',
+          'is_rebuilding' => FALSE,
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::layout',
+          '_title_callback' => '\Drupal\layout_builder\Controller\LayoutBuilderController::title',
+        ],
+        [
+          '_field_ui_view_mode_access' => 'administer with_bundle_parameter display',
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+          ],
+          '_layout_builder' => TRUE,
+          '_admin_route' => FALSE,
+        ]
+      ),
+      'layout_builder.defaults.with_bundle_parameter.save' => new Route(
+        '/admin/entity/{bundle}/display-layout/{view_mode_name}/save',
+        [
+          'entity_type_id' => 'with_bundle_parameter',
+          'section_storage_type' => 'defaults',
+          'section_storage' => '',
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::saveLayout',
+        ],
+        [
+          '_field_ui_view_mode_access' => 'administer with_bundle_parameter display',
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+          ],
+          '_layout_builder' => TRUE,
+          '_admin_route' => FALSE,
+        ]
+      ),
+      'layout_builder.defaults.with_bundle_parameter.cancel' => new Route(
+        '/admin/entity/{bundle}/display-layout/{view_mode_name}/cancel',
+        [
+          'entity_type_id' => 'with_bundle_parameter',
+          'section_storage_type' => 'defaults',
+          'section_storage' => '',
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::cancelLayout',
+        ],
+        [
+          '_field_ui_view_mode_access' => 'administer with_bundle_parameter display',
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+          ],
+          '_layout_builder' => TRUE,
+          '_admin_route' => FALSE,
+        ]
+      ),
+    ];
+
+    $collection = new RouteCollection();
+    $collection->add('known', new Route('/admin/entity/whatever', [], [], ['_admin_route' => TRUE]));
+    $collection->add('with_bundle', new Route('/admin/entity/{bundle}'));
+
+    $this->plugin->buildRoutes($collection);
+    $this->assertEquals($expected, $collection->all());
+    $this->assertSame(array_keys($expected), array_keys($collection->all()));
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/LayoutBuilderRouteEnhancerTest.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\layout_builder\Routing\LayoutBuilderRouteEnhancer;
+use Drupal\Tests\UnitTestCase;
+use Symfony\Cmf\Component\Routing\RouteObjectInterface;
+use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\Routing\Route;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Routing\LayoutBuilderRouteEnhancer
+ * @group layout_builder
+ */
+class LayoutBuilderRouteEnhancerTest extends UnitTestCase {
+
+  /**
+   * @covers ::enhance
+   */
+  public function testEnhanceValidDefaults() {
+    $route = new Route('/the/path', [], [], ['_layout_builder' => TRUE]);
+    $route_enhancer = new LayoutBuilderRouteEnhancer();
+    $defaults = [
+      RouteObjectInterface::ROUTE_OBJECT => $route,
+    ];
+    // Ensure that the 'section_storage' key now contains the value stored for a
+    // given entity type.
+    $expected = [
+      RouteObjectInterface::ROUTE_OBJECT => $route,
+      'is_rebuilding' => TRUE,
+    ];
+    $result = $route_enhancer->enhance($defaults, new Request(['layout_is_rebuilding' => TRUE]));
+    $this->assertEquals($expected, $result);
+
+    $expected['is_rebuilding'] = FALSE;
+    $result = $route_enhancer->enhance($defaults, new Request());
+    $this->assertEquals($expected, $result);
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/LayoutBuilderRoutesTest.php

@@ -0,0 +1,84 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\Core\Routing\RouteBuildEvent;
+use Drupal\layout_builder\Routing\LayoutBuilderRoutes;
+use Drupal\layout_builder\SectionStorage\SectionStorageDefinition;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+use Symfony\Component\Routing\Route;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Routing\LayoutBuilderRoutes
+ *
+ * @group layout_builder
+ */
+class LayoutBuilderRoutesTest extends UnitTestCase {
+
+  /**
+   * The Layout Builder route builder.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface
+   */
+  protected $sectionStorageManager;
+
+  /**
+   * The Layout Builder route builder.
+   *
+   * @var \Drupal\layout_builder\Routing\LayoutBuilderRoutes
+   */
+  protected $routeBuilder;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->sectionStorageManager = $this->prophesize(SectionStorageManagerInterface::class);
+    $this->routeBuilder = new LayoutBuilderRoutes($this->sectionStorageManager->reveal());
+  }
+
+  /**
+   * @covers ::onAlterRoutes
+   */
+  public function testOnAlterRoutes() {
+    $expected = [
+      'test_route1' => new Route('/test/path1'),
+      'test_route_shared' => new Route('/test/path/shared2'),
+      'test_route2' => new Route('/test/path2'),
+    ];
+
+    $section_storage_first = $this->prophesize(SectionStorageInterface::class);
+    $section_storage_first->buildRoutes(Argument::type(RouteCollection::class))->shouldBeCalled()->will(function ($args) {
+      /** @var \Symfony\Component\Routing\RouteCollection $collection */
+      $collection = $args[0];
+      $collection->add('test_route_shared', new Route('/test/path/shared1'));
+      $collection->add('test_route1', new Route('/test/path1'));
+    });
+    $section_storage_second = $this->prophesize(SectionStorageInterface::class);
+    $section_storage_second->buildRoutes(Argument::type(RouteCollection::class))->shouldBeCalled()->will(function ($args) {
+      /** @var \Symfony\Component\Routing\RouteCollection $collection */
+      $collection = $args[0];
+      $collection->add('test_route_shared', new Route('/test/path/shared2'));
+      $collection->add('test_route2', new Route('/test/path2'));
+    });
+
+    $this->sectionStorageManager->loadEmpty('first')->willReturn($section_storage_first->reveal());
+    $this->sectionStorageManager->loadEmpty('second')->willReturn($section_storage_second->reveal());
+    $definitions['first'] = new SectionStorageDefinition();
+    $definitions['second'] = new SectionStorageDefinition();
+    $this->sectionStorageManager->getDefinitions()->willReturn($definitions);
+
+    $collection = new RouteCollection();
+    $event = new RouteBuildEvent($collection);
+    $this->routeBuilder->onAlterRoutes($event);
+    $this->assertEquals($expected, $collection->all());
+    $this->assertSame(array_keys($expected), array_keys($collection->all()));
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/LayoutTempstoreParamConverterTest.php

@@ -0,0 +1,85 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\Routing\LayoutTempstoreParamConverter;
+use Drupal\layout_builder\SectionStorage\SectionStorageManagerInterface;
+use Drupal\layout_builder\SectionStorageInterface;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Routing\LayoutTempstoreParamConverter
+ *
+ * @group layout_builder
+ */
+class LayoutTempstoreParamConverterTest extends UnitTestCase {
+
+  /**
+   * @covers ::convert
+   */
+  public function testConvert() {
+    $layout_tempstore_repository = $this->prophesize(LayoutTempstoreRepositoryInterface::class);
+    $section_storage_manager = $this->prophesize(SectionStorageManagerInterface::class);
+    $converter = new LayoutTempstoreParamConverter($layout_tempstore_repository->reveal(), $section_storage_manager->reveal());
+
+    $section_storage = $this->prophesize(SectionStorageInterface::class);
+
+    $value = 'some_value';
+    $definition = ['layout_builder_tempstore' => TRUE];
+    $name = 'the_parameter_name';
+    $defaults = ['section_storage_type' => 'my_type'];
+    $expected = 'the_return_value';
+
+    $section_storage_manager->hasDefinition('my_type')->willReturn(TRUE);
+    $section_storage_manager->loadFromRoute('my_type', $value, $definition, $name, $defaults)->willReturn($section_storage);
+
+    $layout_tempstore_repository->get($section_storage->reveal())->willReturn($expected);
+
+    $result = $converter->convert($value, $definition, $name, $defaults);
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * @covers ::convert
+   */
+  public function testConvertNoType() {
+    $layout_tempstore_repository = $this->prophesize(LayoutTempstoreRepositoryInterface::class);
+    $section_storage_manager = $this->prophesize(SectionStorageManagerInterface::class);
+    $converter = new LayoutTempstoreParamConverter($layout_tempstore_repository->reveal(), $section_storage_manager->reveal());
+
+    $value = 'some_value';
+    $definition = ['layout_builder_tempstore' => TRUE];
+    $name = 'the_parameter_name';
+    $defaults = ['section_storage_type' => NULL];
+
+    $section_storage_manager->hasDefinition()->shouldNotBeCalled();
+    $section_storage_manager->loadFromRoute()->shouldNotBeCalled();
+    $layout_tempstore_repository->get()->shouldNotBeCalled();
+
+    $result = $converter->convert($value, $definition, $name, $defaults);
+    $this->assertNull($result);
+  }
+
+  /**
+   * @covers ::convert
+   */
+  public function testConvertInvalidConverter() {
+    $layout_tempstore_repository = $this->prophesize(LayoutTempstoreRepositoryInterface::class);
+    $section_storage_manager = $this->prophesize(SectionStorageManagerInterface::class);
+    $converter = new LayoutTempstoreParamConverter($layout_tempstore_repository->reveal(), $section_storage_manager->reveal());
+
+    $value = 'some_value';
+    $definition = ['layout_builder_tempstore' => TRUE];
+    $name = 'the_parameter_name';
+    $defaults = ['section_storage_type' => 'invalid'];
+
+    $section_storage_manager->hasDefinition('invalid')->willReturn(FALSE);
+    $section_storage_manager->loadFromRoute()->shouldNotBeCalled();
+    $layout_tempstore_repository->get()->shouldNotBeCalled();
+
+    $result = $converter->convert($value, $definition, $name, $defaults);
+    $this->assertNull($result);
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/LayoutTempstoreRepositoryTest.php

@@ -0,0 +1,78 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\Core\TempStore\SharedTempStore;
+use Drupal\Core\TempStore\SharedTempStoreFactory;
+use Drupal\layout_builder\LayoutTempstoreRepository;
+use Drupal\layout_builder\SectionStorageInterface;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\LayoutTempstoreRepository
+ * @group layout_builder
+ */
+class LayoutTempstoreRepositoryTest extends UnitTestCase {
+
+  /**
+   * @covers ::get
+   */
+  public function testGetEmptyTempstore() {
+    $section_storage = $this->prophesize(SectionStorageInterface::class);
+    $section_storage->getStorageType()->willReturn('my_storage_type');
+    $section_storage->getStorageId()->willReturn('my_storage_id');
+
+    $tempstore = $this->prophesize(SharedTempStore::class);
+    $tempstore->get('my_storage_id')->shouldBeCalled();
+
+    $tempstore_factory = $this->prophesize(SharedTempStoreFactory::class);
+    $tempstore_factory->get('layout_builder.section_storage.my_storage_type')->willReturn($tempstore->reveal());
+
+    $repository = new LayoutTempstoreRepository($tempstore_factory->reveal());
+
+    $result = $repository->get($section_storage->reveal());
+    $this->assertSame($section_storage->reveal(), $result);
+  }
+
+  /**
+   * @covers ::get
+   */
+  public function testGetLoadedTempstore() {
+    $section_storage = $this->prophesize(SectionStorageInterface::class);
+    $section_storage->getStorageType()->willReturn('my_storage_type');
+    $section_storage->getStorageId()->willReturn('my_storage_id');
+
+    $tempstore_section_storage = $this->prophesize(SectionStorageInterface::class);
+    $tempstore = $this->prophesize(SharedTempStore::class);
+    $tempstore->get('my_storage_id')->willReturn(['section_storage' => $tempstore_section_storage->reveal()]);
+    $tempstore_factory = $this->prophesize(SharedTempStoreFactory::class);
+    $tempstore_factory->get('layout_builder.section_storage.my_storage_type')->willReturn($tempstore->reveal());
+
+    $repository = new LayoutTempstoreRepository($tempstore_factory->reveal());
+
+    $result = $repository->get($section_storage->reveal());
+    $this->assertSame($tempstore_section_storage->reveal(), $result);
+    $this->assertNotSame($section_storage->reveal(), $result);
+  }
+
+  /**
+   * @covers ::get
+   */
+  public function testGetInvalidEntry() {
+    $section_storage = $this->prophesize(SectionStorageInterface::class);
+    $section_storage->getStorageType()->willReturn('my_storage_type');
+    $section_storage->getStorageId()->willReturn('my_storage_id');
+
+    $tempstore = $this->prophesize(SharedTempStore::class);
+    $tempstore->get('my_storage_id')->willReturn(['section_storage' => 'this_is_not_an_entity']);
+
+    $tempstore_factory = $this->prophesize(SharedTempStoreFactory::class);
+    $tempstore_factory->get('layout_builder.section_storage.my_storage_type')->willReturn($tempstore->reveal());
+
+    $repository = new LayoutTempstoreRepository($tempstore_factory->reveal());
+
+    $this->setExpectedException(\UnexpectedValueException::class, 'The entry with storage type "my_storage_type" and ID "my_storage_id" is invalid');
+    $repository->get($section_storage->reveal());
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/OverridesSectionStorageTest.php

@@ -0,0 +1,382 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage;
+use Drupal\layout_builder\SectionStorage\SectionStorageDefinition;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+use Symfony\Component\Routing\Route;
+use Symfony\Component\Routing\RouteCollection;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage
+ *
+ * @group layout_builder
+ */
+class OverridesSectionStorageTest extends UnitTestCase {
+
+  /**
+   * The plugin.
+   *
+   * @var \Drupal\layout_builder\Plugin\SectionStorage\OverridesSectionStorage
+   */
+  protected $plugin;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->entityTypeManager = $this->prophesize(EntityTypeManagerInterface::class);
+    $this->entityFieldManager = $this->prophesize(EntityFieldManagerInterface::class);
+
+    $definition = new SectionStorageDefinition([
+      'id' => 'overrides',
+      'class' => OverridesSectionStorage::class,
+    ]);
+    $this->plugin = new OverridesSectionStorage([], 'overrides', $definition, $this->entityTypeManager->reveal(), $this->entityFieldManager->reveal());
+  }
+
+  /**
+   * @covers ::extractIdFromRoute
+   *
+   * @dataProvider providerTestExtractIdFromRoute
+   */
+  public function testExtractIdFromRoute($expected, $value, array $defaults) {
+    $result = $this->plugin->extractIdFromRoute($value, [], 'the_parameter_name', $defaults);
+    $this->assertSame($expected, $result);
+  }
+
+  /**
+   * Provides data for ::testExtractIdFromRoute().
+   */
+  public function providerTestExtractIdFromRoute() {
+    $data = [];
+    $data['with value, with layout'] = [
+      'my_entity_type.entity_with_layout',
+      'my_entity_type.entity_with_layout',
+      [],
+    ];
+    $data['with value, without layout'] = [
+      NULL,
+      'my_entity_type',
+      [],
+    ];
+    $data['empty value, populated defaults'] = [
+      'my_entity_type.entity_with_layout',
+      '',
+      [
+        'entity_type_id' => 'my_entity_type',
+        'my_entity_type' => 'entity_with_layout',
+      ],
+    ];
+    $data['empty value, empty defaults'] = [
+      NULL,
+      '',
+      [],
+    ];
+    return $data;
+  }
+
+  /**
+   * @covers ::getSectionListFromId
+   *
+   * @dataProvider providerTestGetSectionListFromId
+   */
+  public function testGetSectionListFromId($success, $expected_entity_type_id, $id) {
+    $defaults['the_parameter_name'] = $id;
+
+    if ($expected_entity_type_id) {
+      $entity_storage = $this->prophesize(EntityStorageInterface::class);
+
+      $entity_without_layout = $this->prophesize(FieldableEntityInterface::class);
+      $entity_without_layout->hasField('layout_builder__layout')->willReturn(FALSE);
+      $entity_without_layout->get('layout_builder__layout')->shouldNotBeCalled();
+      $entity_storage->load('entity_without_layout')->willReturn($entity_without_layout->reveal());
+
+      $entity_with_layout = $this->prophesize(FieldableEntityInterface::class);
+      $entity_with_layout->hasField('layout_builder__layout')->willReturn(TRUE);
+      $entity_with_layout->get('layout_builder__layout')->willReturn('the_return_value');
+      $entity_storage->load('entity_with_layout')->willReturn($entity_with_layout->reveal());
+
+      $this->entityTypeManager->getStorage($expected_entity_type_id)->willReturn($entity_storage->reveal());
+    }
+    else {
+      $this->entityTypeManager->getStorage(Argument::any())->shouldNotBeCalled();
+    }
+
+    if (!$success) {
+      $this->setExpectedException(\InvalidArgumentException::class);
+    }
+
+    $result = $this->plugin->getSectionListFromId($id);
+    if ($success) {
+      $this->assertEquals('the_return_value', $result);
+    }
+  }
+
+  /**
+   * Provides data for ::testGetSectionListFromId().
+   */
+  public function providerTestGetSectionListFromId() {
+    $data = [];
+    $data['with value, with layout'] = [
+      TRUE,
+      'my_entity_type',
+      'my_entity_type.entity_with_layout',
+    ];
+    $data['with value, without layout'] = [
+      FALSE,
+      'my_entity_type',
+      'my_entity_type.entity_without_layout',
+    ];
+    $data['empty value, empty defaults'] = [
+      FALSE,
+      NULL,
+      '',
+    ];
+    return $data;
+  }
+
+  /**
+   * @covers ::buildRoutes
+   * @covers ::hasIntegerId
+   * @covers ::getEntityTypes
+   */
+  public function testBuildRoutes() {
+    $entity_types = [];
+
+    $not_fieldable = $this->prophesize(EntityTypeInterface::class);
+    $not_fieldable->entityClassImplements(FieldableEntityInterface::class)->willReturn(FALSE);
+    $entity_types['not_fieldable'] = $not_fieldable->reveal();
+
+    $no_view_builder = $this->prophesize(EntityTypeInterface::class);
+    $no_view_builder->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $no_view_builder->hasViewBuilderClass()->willReturn(FALSE);
+    $entity_types['no_view_builder'] = $no_view_builder->reveal();
+
+    $no_canonical_link = $this->prophesize(EntityTypeInterface::class);
+    $no_canonical_link->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $no_canonical_link->hasViewBuilderClass()->willReturn(TRUE);
+    $no_canonical_link->hasLinkTemplate('canonical')->willReturn(FALSE);
+    $entity_types['no_canonical_link'] = $no_canonical_link->reveal();
+    $this->entityFieldManager->getFieldStorageDefinitions('no_canonical_link')->shouldNotBeCalled();
+
+    $with_string_id = $this->prophesize(EntityTypeInterface::class);
+    $with_string_id->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $with_string_id->hasViewBuilderClass()->willReturn(TRUE);
+    $with_string_id->hasLinkTemplate('canonical')->willReturn(TRUE);
+    $with_string_id->getLinkTemplate('canonical')->willReturn('/entity/{entity}');
+    $with_string_id->id()->willReturn('with_string_id');
+    $with_string_id->getKey('id')->willReturn('id');
+    $entity_types['with_string_id'] = $with_string_id->reveal();
+    $string_id = $this->prophesize(FieldStorageDefinitionInterface::class);
+    $string_id->getType()->willReturn('string');
+    $this->entityFieldManager->getFieldStorageDefinitions('with_string_id')->willReturn(['id' => $string_id->reveal()]);
+
+    $with_integer_id = $this->prophesize(EntityTypeInterface::class);
+    $with_integer_id->entityClassImplements(FieldableEntityInterface::class)->willReturn(TRUE);
+    $with_integer_id->hasViewBuilderClass()->willReturn(TRUE);
+    $with_integer_id->hasLinkTemplate('canonical')->willReturn(TRUE);
+    $with_integer_id->getLinkTemplate('canonical')->willReturn('/entity/{entity}');
+    $with_integer_id->id()->willReturn('with_integer_id');
+    $with_integer_id->getKey('id')->willReturn('id');
+    $entity_types['with_integer_id'] = $with_integer_id->reveal();
+    $integer_id = $this->prophesize(FieldStorageDefinitionInterface::class);
+    $integer_id->getType()->willReturn('integer');
+    $this->entityFieldManager->getFieldStorageDefinitions('with_integer_id')->willReturn(['id' => $integer_id->reveal()]);
+
+    $this->entityTypeManager->getDefinitions()->willReturn($entity_types);
+
+    $expected = [
+      'layout_builder.overrides.with_string_id.view' => new Route(
+        '/entity/{entity}/layout',
+        [
+          'entity_type_id' => 'with_string_id',
+          'section_storage_type' => 'overrides',
+          'section_storage' => '',
+          'is_rebuilding' => FALSE,
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::layout',
+          '_title_callback' => '\Drupal\layout_builder\Controller\LayoutBuilderController::title',
+        ],
+        [
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+            'with_string_id' => ['type' => 'entity:with_string_id'],
+          ],
+          '_layout_builder' => TRUE,
+        ]
+      ),
+      'layout_builder.overrides.with_string_id.save' => new Route(
+        '/entity/{entity}/layout/save',
+        [
+          'entity_type_id' => 'with_string_id',
+          'section_storage_type' => 'overrides',
+          'section_storage' => '',
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::saveLayout',
+        ],
+        [
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+            'with_string_id' => ['type' => 'entity:with_string_id'],
+          ],
+          '_layout_builder' => TRUE,
+        ]
+      ),
+      'layout_builder.overrides.with_string_id.cancel' => new Route(
+        '/entity/{entity}/layout/cancel',
+        [
+          'entity_type_id' => 'with_string_id',
+          'section_storage_type' => 'overrides',
+          'section_storage' => '',
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::cancelLayout',
+        ],
+        [
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+            'with_string_id' => ['type' => 'entity:with_string_id'],
+          ],
+          '_layout_builder' => TRUE,
+        ]
+      ),
+      'layout_builder.overrides.with_string_id.revert' => new Route(
+        '/entity/{entity}/layout/revert',
+        [
+          'entity_type_id' => 'with_string_id',
+          'section_storage_type' => 'overrides',
+          'section_storage' => '',
+          '_form' => '\Drupal\layout_builder\Form\RevertOverridesForm',
+        ],
+        [
+          '_has_layout_section' => 'true',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+            'with_string_id' => ['type' => 'entity:with_string_id'],
+          ],
+          '_layout_builder' => TRUE,
+        ]
+      ),
+      'layout_builder.overrides.with_integer_id.view' => new Route(
+        '/entity/{entity}/layout',
+        [
+          'entity_type_id' => 'with_integer_id',
+          'section_storage_type' => 'overrides',
+          'section_storage' => '',
+          'is_rebuilding' => FALSE,
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::layout',
+          '_title_callback' => '\Drupal\layout_builder\Controller\LayoutBuilderController::title',
+        ],
+        [
+          '_has_layout_section' => 'true',
+          'with_integer_id' => '\d+',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+            'with_integer_id' => ['type' => 'entity:with_integer_id'],
+          ],
+          '_layout_builder' => TRUE,
+        ]
+      ),
+      'layout_builder.overrides.with_integer_id.save' => new Route(
+        '/entity/{entity}/layout/save',
+        [
+          'entity_type_id' => 'with_integer_id',
+          'section_storage_type' => 'overrides',
+          'section_storage' => '',
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::saveLayout',
+        ],
+        [
+          '_has_layout_section' => 'true',
+          'with_integer_id' => '\d+',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+            'with_integer_id' => ['type' => 'entity:with_integer_id'],
+          ],
+          '_layout_builder' => TRUE,
+        ]
+      ),
+      'layout_builder.overrides.with_integer_id.cancel' => new Route(
+        '/entity/{entity}/layout/cancel',
+        [
+          'entity_type_id' => 'with_integer_id',
+          'section_storage_type' => 'overrides',
+          'section_storage' => '',
+          '_controller' => '\Drupal\layout_builder\Controller\LayoutBuilderController::cancelLayout',
+        ],
+        [
+          '_has_layout_section' => 'true',
+          'with_integer_id' => '\d+',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+            'with_integer_id' => ['type' => 'entity:with_integer_id'],
+          ],
+          '_layout_builder' => TRUE,
+        ]
+      ),
+      'layout_builder.overrides.with_integer_id.revert' => new Route(
+        '/entity/{entity}/layout/revert',
+        [
+          'entity_type_id' => 'with_integer_id',
+          'section_storage_type' => 'overrides',
+          'section_storage' => '',
+          '_form' => '\Drupal\layout_builder\Form\RevertOverridesForm',
+        ],
+        [
+          '_has_layout_section' => 'true',
+          'with_integer_id' => '\d+',
+        ],
+        [
+          'parameters' => [
+            'section_storage' => ['layout_builder_tempstore' => TRUE],
+            'with_integer_id' => ['type' => 'entity:with_integer_id'],
+          ],
+          '_layout_builder' => TRUE,
+        ]
+      ),
+    ];
+
+    $collection = new RouteCollection();
+    $this->plugin->buildRoutes($collection);
+    $this->assertEquals($expected, $collection->all());
+    $this->assertSame(array_keys($expected), array_keys($collection->all()));
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/SectionComponentTest.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\Layout\LayoutInterface;
+use Drupal\Core\Layout\LayoutPluginManagerInterface;
+use Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent;
+use Drupal\layout_builder\LayoutBuilderEvents;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+use Symfony\Component\EventDispatcher\EventDispatcherInterface;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\SectionComponent
+ * @group layout_builder
+ */
+class SectionComponentTest extends UnitTestCase {
+
+  /**
+   * @covers ::toRenderArray
+   */
+  public function testToRenderArray() {
+    $existing_block = $this->prophesize(BlockPluginInterface::class);
+    $existing_block->getPluginId()->willReturn('block_plugin_id');
+
+    $block_manager = $this->prophesize(BlockManagerInterface::class);
+    $block_manager->createInstance('some_block_id', ['id' => 'some_block_id'])->willReturn($existing_block->reveal());
+
+    // Imitate an event subscriber by setting a resulting build on the event.
+    $event_dispatcher = $this->prophesize(EventDispatcherInterface::class);
+    $event_dispatcher
+      ->dispatch(LayoutBuilderEvents::SECTION_COMPONENT_BUILD_RENDER_ARRAY, Argument::type(SectionComponentBuildRenderArrayEvent::class))
+      ->shouldBeCalled()
+      ->will(function ($args) {
+        /** @var \Drupal\layout_builder\Event\SectionComponentBuildRenderArrayEvent $event */
+        $event = $args[1];
+        $event->setBuild(['#markup' => $event->getPlugin()->getPluginId()]);
+        return;
+      });
+
+    $layout_plugin = $this->prophesize(LayoutInterface::class);
+    $layout_plugin->build(Argument::type('array'))->willReturnArgument(0);
+
+    $layout_manager = $this->prophesize(LayoutPluginManagerInterface::class);
+    $layout_manager->createInstance('layout_onecol', [])->willReturn($layout_plugin->reveal());
+
+    $container = new ContainerBuilder();
+    $container->set('plugin.manager.block', $block_manager->reveal());
+    $container->set('event_dispatcher', $event_dispatcher->reveal());
+    $container->set('plugin.manager.core.layout', $layout_manager->reveal());
+    \Drupal::setContainer($container);
+
+    $expected = [
+      '#cache' => [
+        'contexts' => [],
+        'tags' => [],
+        'max-age' => -1,
+      ],
+      '#markup' => 'block_plugin_id',
+    ];
+
+    $component = new SectionComponent('some-uuid', 'some-region', ['id' => 'some_block_id']);
+    $result = $component->toRenderArray();
+    $this->assertEquals($expected, $result);
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/SectionRenderTest.php

@@ -0,0 +1,284 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher;
+use Drupal\Component\Plugin\Exception\PluginException;
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\BlockManagerInterface;
+use Drupal\Core\Block\BlockPluginInterface;
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\DependencyInjection\ContainerBuilder;
+use Drupal\Core\Layout\LayoutDefinition;
+use Drupal\Core\Layout\LayoutInterface;
+use Drupal\Core\Layout\LayoutPluginManagerInterface;
+use Drupal\Core\Plugin\Context\ContextHandlerInterface;
+use Drupal\Core\Plugin\Context\ContextRepositoryInterface;
+use Drupal\Core\Plugin\ContextAwarePluginInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\layout_builder\EventSubscriber\BlockComponentRenderArray;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\Tests\UnitTestCase;
+use Prophecy\Argument;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Section
+ * @group layout_builder
+ */
+class SectionRenderTest extends UnitTestCase {
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $account;
+
+  /**
+   * The block plugin manager.
+   *
+   * @var \Drupal\Core\Block\BlockManagerInterface
+   */
+  protected $blockManager;
+
+  /**
+   * The plugin context handler.
+   *
+   * @var \Drupal\Core\Plugin\Context\ContextHandlerInterface
+   */
+  protected $contextHandler;
+
+  /**
+   * The context manager service.
+   *
+   * @var \Drupal\Core\Plugin\Context\ContextRepositoryInterface
+   */
+  protected $contextRepository;
+
+  /**
+   * The event dispatcher.
+   *
+   * @var \Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher
+   */
+  protected $eventDispatcher;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $layout_plugin_manager = $this->prophesize(LayoutPluginManagerInterface::class);
+    $this->blockManager = $this->prophesize(BlockManagerInterface::class);
+    $this->contextHandler = $this->prophesize(ContextHandlerInterface::class);
+    $this->contextRepository = $this->prophesize(ContextRepositoryInterface::class);
+    // @todo Refactor this into some better tests in https://www.drupal.org/node/2942605.
+    $this->eventDispatcher = (new \ReflectionClass(ContainerAwareEventDispatcher::class))->newInstanceWithoutConstructor();
+
+    $this->account = $this->prophesize(AccountInterface::class);
+    $subscriber = new BlockComponentRenderArray($this->account->reveal());
+    $this->eventDispatcher->addSubscriber($subscriber);
+
+    $layout = $this->prophesize(LayoutInterface::class);
+    $layout->getPluginDefinition()->willReturn(new LayoutDefinition([]));
+    $layout->build(Argument::type('array'))->willReturnArgument(0);
+    $layout_plugin_manager->createInstance('layout_onecol', [])->willReturn($layout->reveal());
+
+    $container = new ContainerBuilder();
+    $container->set('current_user', $this->account->reveal());
+    $container->set('plugin.manager.block', $this->blockManager->reveal());
+    $container->set('plugin.manager.core.layout', $layout_plugin_manager->reveal());
+    $container->set('context.handler', $this->contextHandler->reveal());
+    $container->set('context.repository', $this->contextRepository->reveal());
+    $container->set('event_dispatcher', $this->eventDispatcher);
+    \Drupal::setContainer($container);
+  }
+
+  /**
+   * @covers ::toRenderArray
+   */
+  public function testToRenderArray() {
+    $block_content = ['#markup' => 'The block content.'];
+    $render_array = [
+      '#theme' => 'block',
+      '#weight' => 0,
+      '#configuration' => [],
+      '#plugin_id' => 'block_plugin_id',
+      '#base_plugin_id' => 'block_plugin_id',
+      '#derivative_plugin_id' => NULL,
+      'content' => $block_content,
+      '#cache' => [
+        'contexts' => [],
+        'tags' => [],
+        'max-age' => -1,
+      ],
+    ];
+
+    $block = $this->prophesize(BlockPluginInterface::class);
+    $this->blockManager->createInstance('block_plugin_id', ['id' => 'block_plugin_id'])->willReturn($block->reveal());
+
+    $access_result = AccessResult::allowed();
+    $block->access($this->account->reveal(), TRUE)->willReturn($access_result);
+    $block->build()->willReturn($block_content);
+    $block->getCacheContexts()->willReturn([]);
+    $block->getCacheTags()->willReturn([]);
+    $block->getCacheMaxAge()->willReturn(Cache::PERMANENT);
+    $block->getPluginId()->willReturn('block_plugin_id');
+    $block->getBaseId()->willReturn('block_plugin_id');
+    $block->getDerivativeId()->willReturn(NULL);
+    $block->getConfiguration()->willReturn([]);
+
+    $section = [
+      new SectionComponent('some_uuid', 'content', ['id' => 'block_plugin_id']),
+    ];
+    $expected = [
+      'content' => [
+        'some_uuid' => $render_array,
+      ],
+    ];
+    $result = (new Section('layout_onecol', [], $section))->toRenderArray();
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * @covers ::toRenderArray
+   */
+  public function testToRenderArrayAccessDenied() {
+    $block = $this->prophesize(BlockPluginInterface::class);
+    $this->blockManager->createInstance('block_plugin_id', ['id' => 'block_plugin_id'])->willReturn($block->reveal());
+
+    $access_result = AccessResult::forbidden();
+    $block->access($this->account->reveal(), TRUE)->willReturn($access_result);
+    $block->build()->shouldNotBeCalled();
+    $block->getCacheContexts()->willReturn([]);
+    $block->getCacheTags()->willReturn([]);
+    $block->getCacheMaxAge()->willReturn(Cache::PERMANENT);
+
+    $section = [
+      new SectionComponent('some_uuid', 'content', ['id' => 'block_plugin_id']),
+    ];
+    $expected = [
+      'content' => [
+        'some_uuid' => [
+          '#cache' => [
+            'contexts' => [],
+            'tags' => [],
+            'max-age' => -1,
+          ],
+        ],
+      ],
+    ];
+    $result = (new Section('layout_onecol', [], $section))->toRenderArray();
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * @covers ::toRenderArray
+   */
+  public function testToRenderArrayPreview() {
+    $block_content = ['#markup' => 'The block content.'];
+    $render_array = [
+      '#theme' => 'block',
+      '#weight' => 0,
+      '#configuration' => [],
+      '#plugin_id' => 'block_plugin_id',
+      '#base_plugin_id' => 'block_plugin_id',
+      '#derivative_plugin_id' => NULL,
+      'content' => $block_content,
+      '#cache' => [
+        'contexts' => [],
+        'tags' => [],
+        'max-age' => 0,
+      ],
+    ];
+    $block = $this->prophesize(BlockPluginInterface::class);
+    $this->blockManager->createInstance('block_plugin_id', ['id' => 'block_plugin_id'])->willReturn($block->reveal());
+
+    $block->access($this->account->reveal(), TRUE)->shouldNotBeCalled();
+    $block->build()->willReturn($block_content);
+    $block->getCacheContexts()->willReturn([]);
+    $block->getCacheTags()->willReturn([]);
+    $block->getCacheMaxAge()->willReturn(Cache::PERMANENT);
+    $block->getConfiguration()->willReturn([]);
+    $block->getPluginId()->willReturn('block_plugin_id');
+    $block->getBaseId()->willReturn('block_plugin_id');
+    $block->getDerivativeId()->willReturn(NULL);
+
+    $section = [
+      new SectionComponent('some_uuid', 'content', ['id' => 'block_plugin_id']),
+    ];
+    $expected = [
+      'content' => [
+        'some_uuid' => $render_array,
+      ],
+    ];
+    $result = (new Section('layout_onecol', [], $section))->toRenderArray([], TRUE);
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * @covers ::toRenderArray
+   */
+  public function testToRenderArrayEmpty() {
+    $section = [];
+    $expected = [];
+    $result = (new Section('layout_onecol', [], $section))->toRenderArray();
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * @covers ::toRenderArray
+   */
+  public function testContextAwareBlock() {
+    $render_array = [
+      '#theme' => 'block',
+      '#weight' => 0,
+      '#configuration' => [],
+      '#plugin_id' => 'block_plugin_id',
+      '#base_plugin_id' => 'block_plugin_id',
+      '#derivative_plugin_id' => NULL,
+      'content' => [],
+      '#cache' => [
+        'contexts' => [],
+        'tags' => [],
+        'max-age' => -1,
+      ],
+    ];
+
+    $block = $this->prophesize(BlockPluginInterface::class)->willImplement(ContextAwarePluginInterface::class);
+    $this->blockManager->createInstance('block_plugin_id', ['id' => 'block_plugin_id'])->willReturn($block->reveal());
+
+    $access_result = AccessResult::allowed();
+    $block->access($this->account->reveal(), TRUE)->willReturn($access_result);
+    $block->build()->willReturn([]);
+    $block->getCacheContexts()->willReturn([]);
+    $block->getCacheTags()->willReturn([]);
+    $block->getCacheMaxAge()->willReturn(Cache::PERMANENT);
+    $block->getContextMapping()->willReturn([]);
+    $block->getPluginId()->willReturn('block_plugin_id');
+    $block->getBaseId()->willReturn('block_plugin_id');
+    $block->getDerivativeId()->willReturn(NULL);
+    $block->getConfiguration()->willReturn([]);
+
+    $section = [
+      new SectionComponent('some_uuid', 'content', ['id' => 'block_plugin_id']),
+    ];
+    $expected = [
+      'content' => [
+        'some_uuid' => $render_array,
+      ],
+    ];
+    $result = (new Section('layout_onecol', [], $section))->toRenderArray();
+    $this->assertEquals($expected, $result);
+  }
+
+  /**
+   * @covers ::toRenderArray
+   */
+  public function testToRenderArrayMissingPluginId() {
+    $this->setExpectedException(PluginException::class, 'No plugin ID specified for component with "some_uuid" UUID');
+    (new Section('layout_onecol', [], [new SectionComponent('some_uuid', 'content')]))->toRenderArray();
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/SectionStorageManagerTest.php

@@ -0,0 +1,100 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\Component\Plugin\Factory\FactoryInterface;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\layout_builder\SectionListInterface;
+use Drupal\layout_builder\SectionStorage\SectionStorageManager;
+use Drupal\layout_builder\SectionStorageInterface;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\SectionStorage\SectionStorageManager
+ *
+ * @group layout_builder
+ */
+class SectionStorageManagerTest extends UnitTestCase {
+
+  /**
+   * The section storage manager.
+   *
+   * @var \Drupal\layout_builder\SectionStorage\SectionStorageManager
+   */
+  protected $manager;
+
+  /**
+   * The plugin.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $plugin;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $cache = $this->prophesize(CacheBackendInterface::class);
+    $module_handler = $this->prophesize(ModuleHandlerInterface::class);
+    $this->manager = new SectionStorageManager(new \ArrayObject(), $cache->reveal(), $module_handler->reveal());
+
+    $this->plugin = $this->prophesize(SectionStorageInterface::class);
+
+    $factory = $this->prophesize(FactoryInterface::class);
+    $factory->createInstance('the_plugin_id', [])->willReturn($this->plugin->reveal());
+    $reflection_property = new \ReflectionProperty($this->manager, 'factory');
+    $reflection_property->setAccessible(TRUE);
+    $reflection_property->setValue($this->manager, $factory->reveal());
+  }
+
+  /**
+   * @covers ::loadEmpty
+   */
+  public function testLoadEmpty() {
+    $result = $this->manager->loadEmpty('the_plugin_id');
+    $this->assertInstanceOf(SectionStorageInterface::class, $result);
+  }
+
+  /**
+   * @covers ::loadFromStorageId
+   */
+  public function testLoadFromStorageId() {
+    $section_list = $this->prophesize(SectionListInterface::class);
+    $this->plugin->setSectionList($section_list->reveal())->will(function () {
+      return $this;
+    });
+    $this->plugin->getSectionListFromId('the_storage_id')->willReturn($section_list->reveal());
+
+    $result = $this->manager->loadFromStorageId('the_plugin_id', 'the_storage_id');
+    $this->assertInstanceOf(SectionStorageInterface::class, $result);
+  }
+
+  /**
+   * @covers ::loadFromRoute
+   */
+  public function testLoadFromRoute() {
+    $section_list = $this->prophesize(SectionListInterface::class);
+    $this->plugin->extractIdFromRoute('the_value', [], 'the_parameter_name', [])->willReturn('the_storage_id');
+    $this->plugin->getSectionListFromId('the_storage_id')->willReturn($section_list->reveal());
+    $this->plugin->setSectionList($section_list->reveal())->will(function () {
+      return $this;
+    });
+
+    $result = $this->manager->loadFromRoute('the_plugin_id', 'the_value', [], 'the_parameter_name', []);
+    $this->assertInstanceOf(SectionStorageInterface::class, $result);
+  }
+
+  /**
+   * @covers ::loadFromRoute
+   */
+  public function testLoadFromRouteNull() {
+    $this->plugin->extractIdFromRoute('the_value', [], 'the_parameter_name', ['_route' => 'the_route_name'])->willReturn(NULL);
+
+    $result = $this->manager->loadFromRoute('the_plugin_id', 'the_value', [], 'the_parameter_name', ['_route' => 'the_route_name']);
+    $this->assertNull($result);
+  }
+
+}

core/modules/layout_builder/tests/src/Unit/SectionTest.php

@@ -0,0 +1,182 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Unit;
+
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionComponent;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\layout_builder\Section
+ * @group layout_builder
+ */
+class SectionTest extends UnitTestCase {
+
+  /**
+   * The section object to test.
+   *
+   * @var \Drupal\layout_builder\Section
+   */
+  protected $section;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->section = new Section('layout_onecol', [], [
+      new SectionComponent('existing-uuid', 'some-region', ['id' => 'existing-block-id']),
+      (new SectionComponent('second-uuid', 'ordered-region', ['id' => 'second-block-id']))->setWeight(3),
+      (new SectionComponent('first-uuid', 'ordered-region', ['id' => 'first-block-id']))->setWeight(2),
+    ]);
+  }
+
+  /**
+   * @covers ::__construct
+   * @covers ::setComponent
+   * @covers ::getComponents
+   */
+  public function testGetComponents() {
+    $expected = [
+      'existing-uuid' => (new SectionComponent('existing-uuid', 'some-region', ['id' => 'existing-block-id']))->setWeight(0),
+      'second-uuid' => (new SectionComponent('second-uuid', 'ordered-region', ['id' => 'second-block-id']))->setWeight(3),
+      'first-uuid' => (new SectionComponent('first-uuid', 'ordered-region', ['id' => 'first-block-id']))->setWeight(2),
+    ];
+
+    $this->assertComponents($expected, $this->section);
+  }
+
+  /**
+   * @covers ::getComponent
+   */
+  public function testGetComponentInvalidUuid() {
+    $this->setExpectedException(\InvalidArgumentException::class, 'Invalid UUID "invalid-uuid"');
+    $this->section->getComponent('invalid-uuid');
+  }
+
+  /**
+   * @covers ::getComponent
+   */
+  public function testGetComponent() {
+    $expected = new SectionComponent('existing-uuid', 'some-region', ['id' => 'existing-block-id']);
+
+    $this->assertEquals($expected, $this->section->getComponent('existing-uuid'));
+  }
+
+  /**
+   * @covers ::removeComponent
+   * @covers ::getComponentsByRegion
+   */
+  public function testRemoveComponent() {
+    $expected = [
+      'existing-uuid' => (new SectionComponent('existing-uuid', 'some-region', ['id' => 'existing-block-id']))->setWeight(0),
+      'second-uuid' => (new SectionComponent('second-uuid', 'ordered-region', ['id' => 'second-block-id']))->setWeight(3),
+    ];
+
+    $this->section->removeComponent('first-uuid');
+    $this->assertComponents($expected, $this->section);
+  }
+
+  /**
+   * @covers ::appendComponent
+   * @covers ::getNextHighestWeight
+   * @covers ::getComponentsByRegion
+   */
+  public function testAppendComponent() {
+    $expected = [
+      'existing-uuid' => (new SectionComponent('existing-uuid', 'some-region', ['id' => 'existing-block-id']))->setWeight(0),
+      'second-uuid' => (new SectionComponent('second-uuid', 'ordered-region', ['id' => 'second-block-id']))->setWeight(3),
+      'first-uuid' => (new SectionComponent('first-uuid', 'ordered-region', ['id' => 'first-block-id']))->setWeight(2),
+      'new-uuid' => (new SectionComponent('new-uuid', 'some-region', []))->setWeight(1),
+    ];
+
+    $this->section->appendComponent(new SectionComponent('new-uuid', 'some-region'));
+    $this->assertComponents($expected, $this->section);
+  }
+
+  /**
+   * @covers ::insertAfterComponent
+   */
+  public function testInsertAfterComponent() {
+    $expected = [
+      'existing-uuid' => (new SectionComponent('existing-uuid', 'some-region', ['id' => 'existing-block-id']))->setWeight(0),
+      'second-uuid' => (new SectionComponent('second-uuid', 'ordered-region', ['id' => 'second-block-id']))->setWeight(4),
+      'first-uuid' => (new SectionComponent('first-uuid', 'ordered-region', ['id' => 'first-block-id']))->setWeight(2),
+      'new-uuid' => (new SectionComponent('new-uuid', 'ordered-region', []))->setWeight(3),
+    ];
+
+    $this->section->insertAfterComponent('first-uuid', new SectionComponent('new-uuid', 'ordered-region'));
+    $this->assertComponents($expected, $this->section);
+  }
+
+  /**
+   * @covers ::insertAfterComponent
+   */
+  public function testInsertAfterComponentValidUuidRegionMismatch() {
+    $this->setExpectedException(\InvalidArgumentException::class, 'Invalid preceding UUID "existing-uuid"');
+    $this->section->insertAfterComponent('existing-uuid', new SectionComponent('new-uuid', 'ordered-region'));
+  }
+
+  /**
+   * @covers ::insertAfterComponent
+   */
+  public function testInsertAfterComponentInvalidUuid() {
+    $this->setExpectedException(\InvalidArgumentException::class, 'Invalid preceding UUID "invalid-uuid"');
+    $this->section->insertAfterComponent('invalid-uuid', new SectionComponent('new-uuid', 'ordered-region'));
+  }
+
+  /**
+   * @covers ::insertComponent
+   * @covers ::getComponentsByRegion
+   */
+  public function testInsertComponent() {
+    $expected = [
+      'existing-uuid' => (new SectionComponent('existing-uuid', 'some-region', ['id' => 'existing-block-id']))->setWeight(0),
+      'second-uuid' => (new SectionComponent('second-uuid', 'ordered-region', ['id' => 'second-block-id']))->setWeight(4),
+      'first-uuid' => (new SectionComponent('first-uuid', 'ordered-region', ['id' => 'first-block-id']))->setWeight(3),
+      'new-uuid' => (new SectionComponent('new-uuid', 'ordered-region', []))->setWeight(2),
+    ];
+
+    $this->section->insertComponent(0, new SectionComponent('new-uuid', 'ordered-region'));
+    $this->assertComponents($expected, $this->section);
+  }
+
+  /**
+   * @covers ::insertComponent
+   */
+  public function testInsertComponentAppend() {
+    $expected = [
+      'existing-uuid' => (new SectionComponent('existing-uuid', 'some-region', ['id' => 'existing-block-id']))->setWeight(0),
+      'second-uuid' => (new SectionComponent('second-uuid', 'ordered-region', ['id' => 'second-block-id']))->setWeight(3),
+      'first-uuid' => (new SectionComponent('first-uuid', 'ordered-region', ['id' => 'first-block-id']))->setWeight(2),
+      'new-uuid' => (new SectionComponent('new-uuid', 'ordered-region', []))->setWeight(4),
+    ];
+
+    $this->section->insertComponent(2, new SectionComponent('new-uuid', 'ordered-region'));
+    $this->assertComponents($expected, $this->section);
+  }
+
+  /**
+   * @covers ::insertComponent
+   */
+  public function testInsertComponentInvalidDelta() {
+    $this->setExpectedException(\OutOfBoundsException::class, 'Invalid delta "7" for the "new-uuid" component');
+    $this->section->insertComponent(7, new SectionComponent('new-uuid', 'ordered-region'));
+  }
+
+  /**
+   * Asserts that the section has the expected components.
+   *
+   * @param \Drupal\layout_builder\SectionComponent[] $expected
+   *   The expected sections.
+   * @param \Drupal\layout_builder\Section $section
+   *   The section storage to check.
+   */
+  protected function assertComponents(array $expected, Section $section) {
+    $result = $section->getComponents();
+    $this->assertEquals($expected, $result);
+    $this->assertSame(array_keys($expected), array_keys($result));
+  }
+
+}