TabbableShimTestController.php

<?php

namespace Drupal\tabbable_shim_test\Controller;

use Drupal\Core\Controller\ControllerBase;

/**
 * For testing the jQuery :tabbable shim.
 */
class TabbableShimTestController extends ControllerBase {
    
    /**
     * Provides a page with the tabbingManager library for testing :tabbable.
     *
     * @return array
     *   The render array.
     */
    public function build() {
        return [
            'container' => [
                '#type' => 'container',
                '#attributes' => [
                    'id' => 'tabbable-test-container',
                ],
            ],
            '#attached' => [
                'library' => [
                    'core/jquery.ui',
                ],
            ],
        ];
    }

}