Issue #3219340 by lauriii, bnjmnm, mherchel: Vertical tabs with #parents are broken in Claro

(cherry picked from commit cbf7b552)

core/tests/Drupal/KernelTests/Core/Theme/ClaroVerticalTabsTest.php

@@ -26,9 +26,9 @@ public function testVerticalTabs() {
     $this->config('system.theme')->set('default', 'claro')->save();
 
     $form = [
-      '#parents' => [],
+      '#parents' => ['parent'],
       '#array_parents' => [],
-      '#tree' => FALSE,
+      '#tree' => TRUE,
     ];
 
     $form['vertical_tabs'] = [
@@ -38,7 +38,7 @@ public function testVerticalTabs() {
     $form['vertical_tabs_details'] = [
       '#type' => 'details',
       '#title' => 'Details',
-      '#group' => 'vertical_tabs',
+      '#group' => 'parent][vertical_tabs',
     ];
 
     // Needs to be rendered after the vertical tabs.
@@ -61,9 +61,9 @@ public function testVerticalTabs() {
     // Assert that the vertical tab details has the appropriate class.
     $this->assertCount(1, $this->cssSelect('.vertical-tabs__items details.vertical-tabs__item'));
     // Assert that there is a details element.
-    $this->assertCount(1, $this->cssSelect('#edit-container-details'));
+    $this->assertCount(1, $this->cssSelect('#edit-parent-container-details'));
     // Assert that details element doesn't have the vertical tab classes.
-    $this->assertCount(0, $this->cssSelect('#edit-container-details.vertical-tabs__item'));
+    $this->assertCount(0, $this->cssSelect('#edit-parent-container-details.vertical-tabs__item'));
   }
 
 }

core/themes/claro/src/ClaroPreRender.php

@@ -77,13 +77,10 @@ public static function verticalTabs($element) {
       $last_group_with_child_key = NULL;
       $last_group_with_child_key_last_child_key = NULL;
 
-      // Only iterate through the parents instead of all the group keys.
-      foreach ($element['#parents'] as $group_key) {
-        // Check parents against groups because we are only looking for group
-        // elements.
-        if (!in_array($group_key, $group_keys)) {
-          continue;
-        }
+      $group_key = implode('][', $element['#parents']);
+      // Only check siblings against groups because we are only looking for
+      // group elements.
+      if (in_array($group_key, $group_keys)) {
         $children_keys = Element::children($element['group']['#groups'][$group_key], TRUE);
 
         foreach ($children_keys as $child_key) {