Issue #3227370 by pfrenssen: Toolbar menu theme override omits the 'menu_name' variable

(cherry picked from commit 117db6f91b)

core/modules/toolbar/tests/modules/toolbar_test/toolbar_test.module

@@ -48,3 +48,14 @@ function toolbar_test_toolbar() {
 
   return $items;
 }
+
+/**
+ * Implements hook_preprocess_HOOK().
+ */
+function toolbar_test_preprocess_menu(&$variables) {
+  // All the standard hook_theme variables should be populated when the
+  // Toolbar module is rendering a menu.
+  foreach (['menu_name', 'items', 'attributes'] as $variable) {
+    $variables[$variable];
+  }
+}

core/modules/toolbar/toolbar.module

@@ -41,7 +41,7 @@ function toolbar_theme($existing, $type, $theme, $path) {
   ];
   $items['menu__toolbar'] = [
     'base hook' => 'menu',
-    'variables' => ['items' => [], 'attributes' => []],
+    'variables' => ['menu_name' => NULL, 'items' => [], 'attributes' => []],
   ];
 
   return $items;