Issue #3509402 by tocab, grimreaper, laetitia_al: Fix CKE5 z-index for modal integration.

assets/css/ckeditor5/ckeditor5-bootstrap.css

+/**
+ * CKEditor5 Bootstrap compatibility.
+ *
+ * @see https://ckeditor.com/docs/ckeditor5/latest/getting-started/installation/css.html#bootstrap-modals
+ */
+
+/**
+ * Configure the z-index of the editor UI, so when inside a Bootstrap
+ * modal, it will be rendered over the modal (for example, balloons).
+ */
+:root {
+  --ck-z-default: 100 !important;
+}

assets/css/ckeditor5/ckeditor5-theme.css

 /**
- * CKEditor5 theme override, this can be enabled with the ui_skins module
- * in this theme settings.
+ * CKEditor5 theme override.
+ *
+ * The theme selection is possible with the UI Skins module in this theme
+ * settings.
+ *
  * @see https://ckeditor.com/docs/ckeditor5/latest/framework/deep-dive/ui/theme-customization.html
  */
 [data-bs-theme="light"],

ui_suite_bootstrap.info.yml

@@ -29,6 +29,7 @@ libraries:
   - ui_suite_bootstrap/form
 
 ckeditor5-stylesheets:
+  - assets/css/ckeditor5/ckeditor5-bootstrap.css
   - assets/css/ckeditor5/ckeditor5-theme.css
 
 libraries-override: