Issue #2567091 by slashrsm: AJAX updates of an element in a #group are not working

core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php

@@ -81,6 +81,15 @@ public function buildResponse(Request $request, array $form, FormStateInterface
       $response = $result;
     }
     else {
+      // At this point we know callback returned a render element. If the
+      // element is part of the group (#group is set on it) it won't be rendered
+      // unless we remove #group from it. This is caused by
+      // \Drupal\Core\Render\Element\RenderElement::preRenderGroup(), which
+      // prevents all members of groups from being rendered directly.
+      if (!empty($result['#group'])) {
+        unset($result['#group']);
+      }
+
       /** @var \Drupal\Core\Ajax\AjaxResponse $response */
       $response = $this->ajaxRenderer->renderResponse($result, $request, $this->routeMatch);
     }

core/modules/system/src/Tests/Ajax/AjaxInGroupTest.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Ajax\AjaxInGroupTest.
+ */
+
+namespace Drupal\system\Tests\Ajax;
+
+use Drupal\Core\Ajax\DataCommand;
+
+/**
+ * Tests that form elements in groups work correctly with AJAX.
+ *
+ * @group Ajax
+ */
+class AjaxInGroupTest extends AjaxTestBase {
+  protected function setUp() {
+    parent::setUp();
+
+    $this->drupalLogin($this->drupalCreateUser(array('access content')));
+  }
+
+  /**
+   * Submits forms with select and checkbox elements via Ajax.
+   */
+  function testSimpleAjaxFormValue() {
+    $this->drupalGet('/ajax_forms_test_get_form');
+    $this->assertText('Test group');
+    $this->assertText('AJAX checkbox in a group');
+
+    $this->drupalPostAjaxForm(NULL, ['checkbox_in_group' => TRUE], 'checkbox_in_group');
+    $this->assertText('Test group');
+    $this->assertText('AJAX checkbox in a group');
+    $this->assertText('AJAX checkbox in a nested group');
+    $this->assertText('Another AJAX checkbox in a nested group');
+  }
+}

core/modules/system/tests/modules/ajax_forms_test/src/Callbacks.php

@@ -36,4 +36,12 @@ function checkboxCallback($form, FormStateInterface $form_state) {
     $response->addCommand(new DataCommand('#ajax_checkbox_value', 'form_state_value_select', (int) $form_state->getValue('checkbox')));
     return $response;
   }
+
+  /**
+   * Ajax callback triggered by the checkbox in a #group.
+   */
+  function checkboxGroupCallback($form, FormStateInterface $form_state) {
+    return $form['checkbox_in_group_wrapper'];
+  }
+
 }

core/modules/system/tests/modules/ajax_forms_test/src/Form/AjaxFormsTestSimpleForm.php

@@ -72,6 +72,48 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       );
     }
 
+    $form['test_group'] = [
+      '#type' => 'details',
+      '#title' => $this->t('Test group'),
+      '#open' => TRUE,
+    ];
+
+    // Test ajax element in a #group.
+    $form['checkbox_in_group_wrapper'] = [
+      '#type' => 'container',
+      '#attributes' => ['id' => 'checkbox-wrapper'],
+      '#group' => 'test_group',
+      'checkbox_in_group' => [
+        '#type' => 'checkbox',
+        '#title' => $this->t('AJAX checkbox in a group'),
+        '#ajax' => [
+          'callback' => [$object, 'checkboxGroupCallback'],
+          'wrapper' => 'checkbox-wrapper',
+        ],
+      ],
+      'nested_group' => [
+        '#type' => 'details',
+        '#title' => $this->t('Nested group'),
+        '#open' => TRUE,
+      ],
+      'checkbox_in_nested' => [
+        '#type' => 'checkbox',
+        '#group' => 'nested_group',
+        '#title' => $this->t('AJAX checkbox in a nested group'),
+        '#ajax' => [
+          'callback' => [$object, 'checkboxGroupCallback'],
+          'wrapper' => 'checkbox-wrapper',
+        ],
+      ],
+    ];
+
+    $form['another_checkbox_in_nested'] = [
+      '#type' => 'checkbox',
+      '#group' => 'nested_group',
+      '#title' => $this->t('Another AJAX checkbox in a nested group'),
+    ];
+
+
     return $form;
   }