Issue #3512236 by catch, smustgrave: Move fieldgroup CSS to its own library

core/core.libraries.yml

@@ -589,6 +589,21 @@ drupal.entity-form:
   dependencies:
     - core/drupal.form
 
+drupal.fieldgroup:
+  version: VERSION
+  css:
+    component:
+      misc/components/fieldgroup.module.css: { weight: -10 }
+  moved_files:
+    system/base:
+      deprecation_version: 11.2.0
+      removed_version: 12.0.0
+      deprecation_link: https://www.drupal.org/node/3432346
+      css:
+        component:
+          css/components/fieldgroup.module.css:
+            base: core/misc/components/fieldgroup.module.css
+
 drupal.form:
   version: VERSION
   js:

core/lib/Drupal/Core/Render/Element/Fieldgroup.php

@@ -20,9 +20,18 @@
 class Fieldgroup extends Fieldset {
 
   public function getInfo() {
-    return [
-      '#attributes' => ['class' => ['fieldgroup']],
-    ] + parent::getInfo();
+    $info = parent::getInfo();
+    $info['#attributes']['class'] = ['fieldgroup'];
+    $info['#pre_render'][] = [static::class, 'preRenderAttachments'];
+    return $info;
+  }
+
+  /**
+   * Adds the fieldgroup library.
+   */
+  public static function preRenderAttachments($element): array {
+    $element['#attached']['library'][] = 'core/drupal.fieldgroup';
+    return $element;
   }
 
 }

core/modules/system/system.libraries.yml

@@ -4,7 +4,6 @@ base:
     # Adjust the weights to load these early.
     component:
       css/components/align.module.css: { weight: -10 }
-      css/components/fieldgroup.module.css: { weight: -10 }
       css/components/container-inline.module.css: { weight: -10 }
       css/components/clearfix.module.css: { weight: -10 }
       css/components/details.module.css: { weight: -10 }

core/themes/stable9/stable9.info.yml

@@ -68,6 +68,11 @@ libraries-override:
       component:
         misc/dropbutton/dropbutton.css: css/core/dropbutton/dropbutton.css
 
+  core/drupal.fieldgroup:
+    css:
+      component:
+        misc/components/fieldgroup.module.css: css/core/components/fieldgroup.module.css
+
   core/drupal.progress:
     css:
       component:
@@ -220,7 +225,6 @@ libraries-override:
     css:
       component:
         css/components/align.module.css: css/system/components/align.module.css
-        css/components/fieldgroup.module.css: css/system/components/fieldgroup.module.css
         css/components/container-inline.module.css: css/system/components/container-inline.module.css
         css/components/clearfix.module.css: css/system/components/clearfix.module.css
         css/components/details.module.css: css/system/components/details.module.css