Issue #2361599 by stefan.korn: Fixed menu_ui css file is not loaded.

core/modules/menu_ui/menu_ui.libraries.yml

@@ -14,3 +14,9 @@ drupal.menu_ui.admin:
   dependencies:
     - core/jquery
     - core/drupal
+
+drupal.menu_ui.adminforms:
+  version: VERSION
+  css:
+    theme:
+      css/menu_ui.admin.css: {}

core/modules/menu_ui/src/MenuForm.php

@@ -220,7 +220,7 @@ class MenuForm extends EntityForm {
       $form_state->set('menu_overview_form_parents', []);
     }
 
-    $form['#attached']['css'] = array(drupal_get_path('module', 'menu') . '/css/menu.admin.css');
+    $form['#attached']['library'][] = 'menu_ui/drupal.menu_ui.adminforms';
 
     $tree = $this->menuTree->load($this->entity->id(), new MenuTreeParameters());
 

core/modules/menu_ui/src/MenuListBuilder.php

@@ -68,7 +68,7 @@ class MenuListBuilder extends ConfigEntityListBuilder {
    */
   public function render() {
     $build = parent::render();
-    $build['#attached']['css'][] = drupal_get_path('module', 'menu') . '/css/menu.admin.css';
+    $build['#attached']['library'][] = "menu_ui/drupal.menu_ui.adminforms";
     return $build;
   }