Issue #3173159 by sardara, JeroenT: Block add form ajax callback implementation issues

core/modules/block/src/BlockForm.php

@@ -186,7 +186,7 @@ public function form(array $form, FormStateInterface $form_state) {
       '#description' => $this->t('Select the region where this block should be displayed.'),
       '#default_value' => $region,
       '#required' => TRUE,
-      '#options' => system_region_list($theme, REGIONS_VISIBLE),
+      '#options' => system_region_list($form_state->getValue('theme', $theme), REGIONS_VISIBLE),
       '#prefix' => '<div id="edit-block-region-wrapper">',
       '#suffix' => '</div>',
     ];
@@ -198,7 +198,6 @@ public function form(array $form, FormStateInterface $form_state) {
    * Handles switching the available regions based on the selected theme.
    */
   public function themeSwitch($form, FormStateInterface $form_state) {
-    $form['region']['#options'] = system_region_list($form_state->getValue('theme'), REGIONS_VISIBLE);
     return $form['region'];
   }
 

core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php

+<?php
+
+namespace Drupal\Tests\block\FunctionalJavascript;
+
+use Drupal\FunctionalJavascriptTests\WebDriverTestBase;
+
+/**
+ * Tests the JS functionality in the block add form.
+ *
+ * @group block
+ */
+class BlockAddTest extends WebDriverTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'block',
+    'user',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests the AJAX for the theme selector.
+   */
+  public function testBlockAddThemeSelector() {
+    \Drupal::service('theme_installer')->install(['seven']);
+
+    $this->drupalLogin($this->drupalCreateUser([
+      'administer blocks',
+    ]));
+
+    $this->drupalGet('admin/structure/block/add/system_powered_by_block');
+    $assert_session = $this->assertSession();
+    // Pick a theme with a region that does not exist in another theme.
+    $assert_session->selectExists('Theme')->selectOption('seven');
+    $assert_session->assertWaitOnAjaxRequest();
+    $assert_session->selectExists('Region')->selectOption('pre_content');
+    $assert_session->assertWaitOnAjaxRequest();
+    // Switch to a theme that doesn't contain the region selected above.
+    $assert_session->selectExists('Theme')->selectOption('stark');
+    $assert_session->assertWaitOnAjaxRequest();
+    $assert_session->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $assert_session->optionExists('Region', '- Select -');
+  }
+
+}