switched admin theme to GIN (plus custom module for terme_reference_tree display)

web/modules/custom/materio_admin/css/term-reference-tree.css

@@ -0,0 +1,17 @@
+/**
+ * Term reference tree widget: selection tree on the left, sortable track
+ * list on the right. Transposed from the retired matminimal theme.
+ */
+.term-reference-tree > ul,
+.term-reference-tree aside.term-reference-tree-track-list {
+  display: inline-block;
+  vertical-align: top;
+}
+.term-reference-tree > ul {
+  padding-right: 1em;
+  min-width: 45%;
+}
+.term-reference-tree aside.term-reference-tree-track-list {
+  border-left: 1px solid #aaa;
+  padding-left: 1em;
+}

web/modules/custom/materio_admin/materio_admin.info.yml

@@ -0,0 +1,7 @@
+name: 'Materio Admin'
+type: module
+description: 'Admin back-office enhancements (widget layouts and theming support for the Gin admin theme).'
+package: Materio
+core_version_requirement: ^10 || ^11
+dependencies:
+  - 'term_reference_tree'

web/modules/custom/materio_admin/materio_admin.libraries.yml

@@ -0,0 +1,5 @@
+term_reference_tree:
+  version: 1.0
+  css:
+    theme:
+      css/term-reference-tree.css: {}

web/modules/custom/materio_admin/materio_admin.module

@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * @file
+ * Admin back-office enhancements.
+ */
+
+use Drupal\term_reference_tree\Plugin\Field\FieldWidget\TermReferenceTree;
+
+/**
+ * Implements hook_field_widget_single_element_form_alter().
+ *
+ * Attaches the widget layout library wherever the term reference tree widget
+ * renders (thesaurus on materiau, etc.), so the selection tree (left) and the
+ * sortable track list (right) sit side by side.
+ *
+ * Transposed from the retired matminimal theme CSS, which was tied to the old
+ * adminimal-based admin theme. The selectors target the widget's own markup
+ * classes and are admin-theme agnostic (works under Gin 4 and Gin 5).
+ */
+function materio_admin_field_widget_single_element_form_alter(array &$element, \Drupal\Core\Form\FormStateInterface $form_state, array $context) {
+  if (isset($context['widget']) && $context['widget'] instanceof TermReferenceTree) {
+    $element['#attached']['library'][] = 'materio_admin/term_reference_tree';
+  }
+}