.../tests/modules/ajax_test/src/Form/AjaxTestForm.php      | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/core/modules/system/tests/modules/ajax_test/src/Form/AjaxTestForm.php b/core/modules/system/tests/modules/ajax_test/src/Form/AjaxTestForm.php
index 2b3d1ea..3704f15 100644
--- a/core/modules/system/tests/modules/ajax_test/src/Form/AjaxTestForm.php
+++ b/core/modules/system/tests/modules/ajax_test/src/Form/AjaxTestForm.php
@@ -7,10 +7,8 @@
 
 namespace Drupal\ajax_test\Form;
 
-use Drupal\Core\Ajax\AjaxResponse;
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
-use Symfony\Component\HttpFoundation\Response;
 
 /**
  * Dummy form for testing DialogRenderer with _form routes.
@@ -48,6 +46,14 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       // No regular submit-handler. This form only works via JavaScript.
       '#submit' => array(),
       '#ajax' => array(
+        // This means the ::preview() method on this class would be invoked in
+        // case of a click event. However, since Drupal core's test runner only
+        // is able to execute PHP, not JS, there is no point in actually
+        // implementing this method, because we can never let it be called from
+        // JS; we'd have to manually call it from PHP, at which point we would
+        // not actually be testing it.
+        // Therefore we consciously choose to not implement this method, because
+        // we cannot meaningfully test it anyway.
         'callback' => '::preview',
         'event' => 'click',
       ),
@@ -65,8 +71,4 @@ public function submitForm(array &$form, FormStateInterface $form_state) {}
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {}
 
-  public function preview(array &$form, FormStateInterface $form_state) {
-    return new Response('#ajax-triggered submit worked!');
-  }
-
 }