edlp_d8/core/modules/field_ui/tests/modules/field_ui_test/field_ui_test.module

<?php

/**
 * @file
 * Field UI test module.
 */

use Drupal\Core\Access\AccessResult;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Render\Element;
use Drupal\Core\Field\FieldConfigInterface;

/**
 * Implements hook_ENTITY_TYPE_access().
 */
function field_ui_test_field_config_access(FieldConfigInterface $field) {
  return AccessResult::forbiddenIf($field->getName() == 'highlander');
}

/**
 * Implements hook_form_FORM_BASE_ID_alter().
 */
function field_ui_test_form_entity_view_display_edit_form_alter(&$form, FormStateInterface $form_state) {
  $table = &$form['fields'];

  foreach (Element::children($table) as $name) {
    $table[$name]['parent_wrapper']['parent']['#options'] = ['indent' => 'Indent'];
    $table[$name]['parent_wrapper']['parent']['#default_value'] = 'indent';
  }

  $table['indent'] = [
    '#attributes' => ['class' => ['draggable', 'field-group'], 'id' => 'indent-id'],
    '#row_type' => 'group',
    '#region_callback' => 'field_ui_test_region_callback',
    '#js_settings' => ['rowHandler' => 'group'],
    'human_name' => [
      '#markup' => 'Indent',
      '#prefix' => '<span class="group-label">',
      '#suffix' => '</span>',
    ],
    'weight' => [
      '#type' => 'textfield',
      '#default_value' => 0,
      '#size' => 3,
      '#attributes' => ['class' => ['field-weight']],
    ],
    'parent_wrapper' => [
      'parent' => [
        '#type' => 'select',
        '#options' => ['indent' => 'Indent'],
        '#empty_value' => '',
        '#default_value' => '',
        '#attributes' => ['class' => ['field-parent']],
        '#parents' => ['fields', 'indent', 'parent'],
      ],
      'hidden_name' => [
        '#type' => 'hidden',
        '#default_value' => 'indent',
        '#attributes' => ['class' => ['field-name']],
      ],
    ],
  ];

}

function field_ui_test_region_callback($row) {
  return 'content';
}