FormTestGroupContainerForm.php

<?php

namespace Drupal\form_test\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Builds a simple form to test the #group property on #type 'container'.
 *
 * @internal
 */
class FormTestGroupContainerForm extends FormBase {
    
    /**
     * {@inheritdoc}
     */
    public function getFormId() {
        return 'form_test_group_container';
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state) {
        $form['container'] = [
            '#type' => 'container',
        ];
        $form['meta'] = [
            '#type' => 'details',
            '#title' => 'Group element',
            '#open' => TRUE,
            '#group' => 'container',
        ];
        $form['meta']['element'] = [
            '#type' => 'textfield',
            '#title' => 'Nest in details element',
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
    }

}