.../src/FunctionalJavascript/SourceEditingTest.php | 39 ++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/core/modules/ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php b/core/modules/ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php
index d9a057fcb6..daa76c4851 100644
--- a/core/modules/ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php
+++ b/core/modules/ckeditor5/tests/src/FunctionalJavascript/SourceEditingTest.php
@@ -114,6 +114,45 @@ function (ConstraintViolation $v) {
     $this->drupalLogin($this->adminUser);
   }
 
+  /**
+   * @covers \Drupal\ckeditor5\Plugin\CKEditor5Plugin\SourceEditing::buildConfigurationForm
+   */
+  public function testSourceEditingSettingsForm() {
+    $this->drupalLogin($this->drupalCreateUser(['administer filters']));
+
+    $page = $this->getSession()->getPage();
+    $assert_session = $this->assertSession();
+
+    $this->createNewTextFormat($page, $assert_session);
+    $assert_session->assertWaitOnAjaxRequest();
+
+    // The Source Editing plugin settings form should not be present.
+    $assert_session->elementNotExists('css', '[data-drupal-selector="edit-editor-settings-plugins-ckeditor5-sourceediting"]');
+
+    $this->assertNotEmpty($assert_session->waitForElement('css', '.ckeditor5-toolbar-item-sourceEditing'));
+    $this->triggerKeyUp('.ckeditor5-toolbar-item-sourceEditing', 'ArrowDown');
+    $assert_session->assertWaitOnAjaxRequest();
+
+    // The Source Editing plugin settings form should now be present and should
+    // have no allowed tags configured.
+    $element = $assert_session->elementExists('css', '[data-drupal-selector="edit-editor-settings-plugins-ckeditor5-sourceediting"]');
+    $page->clickLink('Source editing');
+    $source_editing_allowed_tags_textarea = $element->find('css', 'textarea');
+    $this->assertSame('', $source_editing_allowed_tags_textarea->getValue());
+
+    // Configure it.
+    $source_editing_allowed_tags_textarea->setValue('<div data-foo>');
+
+    // Immediately save the configuration.
+    $page->pressButton('Save configuration');
+
+    // Verify that the configuration was saved.
+    $this->drupalGet('admin/config/content/formats/manage/ckeditor5');
+    $page->clickLink('Source editing');
+    $textarea = $element->find('css', 'textarea');
+    $this->assertSame('<div data-foo>', $textarea->getValue());
+  }
+
   /**
    * Tests allowing extra attributes on already supported tags using GHS.
    *