--- /dev/null
+<?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 'details'.
+ */
+class FormTestGroupDetailsForm extends FormBase {
+
+ /**
+ * {@inheritdoc}
+ */
+ public function getFormId() {
+ return 'form_test_group_details';
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function buildForm(array $form, FormStateInterface $form_state, $required = FALSE) {
+ $form['details'] = [
+ '#type' => 'details',
+ '#title' => 'Root element',
+ '#open' => TRUE,
+ '#required' => !empty($required),
+ ];
+ $form['meta'] = [
+ '#type' => 'details',
+ '#title' => 'Group element',
+ '#open' => TRUE,
+ '#group' => 'details',
+ ];
+ $form['meta']['element'] = [
+ '#type' => 'textfield',
+ '#title' => 'Nest in details element',
+ ];
+ return $form;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function submitForm(array &$form, FormStateInterface $form_state) {
+ }
+
+}