--- /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 'container'.
+ */
+class FormTestDetailsForm extends FormBase {
+
+ /**
+ * {@inheritdoc}
+ */
+ public function getFormId() {
+ return 'form_test_details_form';
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function buildForm(array $form, FormStateInterface $form_state) {
+ $form['meta'] = [
+ '#type' => 'details',
+ '#title' => 'Details element',
+ '#open' => TRUE,
+ ];
+ $form['submit'] = ['#type' => 'submit', '#value' => 'Submit'];
+ return $form;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function validateForm(array &$form, FormStateInterface $form_state) {
+ $form_state->setErrorByName('meta', 'I am an error on the details element.');
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function submitForm(array &$form, FormStateInterface $form_state) {
+ }
+
+}