Issue #3256922 by danflanagan8, GaëlG: Warning on AJAX call when changing the breakpoint_group field value of a responsive image style

(cherry picked from commit 9dc6f60e72)

core/modules/responsive_image/src/ResponsiveImageStyleForm.php

@@ -225,6 +225,7 @@ class ResponsiveImageStyleForm extends EntityForm {
       if ($form_state->getValue('breakpoint_group') != $form_state->getCompleteForm()['breakpoint_group']['#default_value']) {
         // Remove the image style mappings since the breakpoint ID has changed.
         $form_state->unsetValue('keyed_styles');
+        return;
       }
 
       // Check that at least 1 image style has been selected when using sizes.

core/modules/responsive_image/tests/src/Functional/ResponsiveImageAdminUITest.php

@@ -55,7 +55,7 @@ class ResponsiveImageAdminUITest extends BrowserTestBase {
     $edit = [
       'label' => 'Style One',
       'id' => 'style_one',
-      'breakpoint_group' => 'responsive_image_test_module',
+      'breakpoint_group' => 'responsive_image',
       'fallback_image_style' => 'thumbnail',
     ];
     $this->drupalGet('admin/config/media/responsive-image-style/add');
@@ -67,6 +67,15 @@ class ResponsiveImageAdminUITest extends BrowserTestBase {
     $this->assertSession()->pageTextNotContains('There are no responsive image styles yet.');
     $this->assertSession()->pageTextContains('Style One');
 
+    // Edit the breakpoint_group.
+    $this->drupalGet('admin/config/media/responsive-image-style/style_one');
+    $this->assertSession()->fieldValueEquals('label', 'Style One');
+    $this->assertSession()->fieldValueEquals('breakpoint_group', 'responsive_image');
+    $edit = [
+      'breakpoint_group' => 'responsive_image_test_module',
+    ];
+    $this->submitForm($edit, 'Save');
+
     // Edit the group.
     $this->drupalGet('admin/config/media/responsive-image-style/style_one');
     $this->assertSession()->fieldValueEquals('label', 'Style One');