Issue #3177415 by ramil g, joelpittet, jplana, kishor_kolekar: Vertical Tabs...

<?php

namespace Drupal\KernelTests\Core\Theme;

use Drupal\KernelTests\KernelTestBase;

use Drupal\Core\Form\FormState;

/**

 * Confirms that Claro can render vertical tabs correctly.

 *

 * @group Theme

 */

class ClaroVerticalTabsTest extends KernelTestBase {

  /**

   * {@inheritdoc}

   */

  protected static $modules = ['system'];

  /**

   * Confirms that Claro can render vertical tabs correctly.

   */

  public function testVerticalTabs() {

    // Enable the Claro theme.

    \Drupal::service('theme_installer')->install(['claro']);

    $this->config('system.theme')->set('default', 'claro')->save();

    $form = [

      '#parents' => [],

      '#array_parents' => [],

      '#tree' => FALSE,

    ];

    $form['vertical_tabs'] = [

      '#type' => 'vertical_tabs',

      '#title' => 'Vertical Tabs',

    ];

    $form['vertical_tabs_details'] = [

      '#type' => 'details',

      '#title' => 'Details',

      '#group' => 'vertical_tabs',

    ];

    // Needs to be rendered after the vertical tabs.

    $form['container'] = [

      '#type' => 'container',

      '#title' => 'Container',

    ];

    $form['container_details'] = [

      '#type' => 'details',

      '#title' => 'Details',

      '#group' => 'container',

    ];

    /** @var \Drupal\Core\Form\FormBuilderInterface $form_builder */

    $form_builder = $this->container->get('form_builder');

    $form_state = new FormState();

    $form_builder->doBuildForm('form', $form, $form_state);

    $this->render($form);

    // 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'));

    // Assert that details element doesn't have the vertical tab classes.

    $this->assertCount(0, $this->cssSelect('#edit-container-details.vertical-tabs__item'));

  }

}

      $last_group_with_child_key = NULL;

      $last_group_with_child_key_last_child_key = NULL;

      foreach ($group_keys as $group_key) {

      // 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;

        }

        $children_keys = Element::children($element['group']['#groups'][$group_key], TRUE);

        foreach ($children_keys as $child_key) {