diff -u b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
@@ -57,18 +57,17 @@
    *   be displayed.
    */
   public function waitForJavascriptActivity($timeout = 10000, $message = 'Unable to complete javascript activity.') {
-    $condition = <<<JS
-      (function() {
-        function DrupalIsAjaxing() {
+    $condition =
+      <<<JS
+            (function() {
+        function IsNotAjaxing(instance, index, array) {
           "use strict";
-          if (Drupal && Drupal.Ajax && Drupal.ajax.instances) {
-            for (var i = Drupal.ajax.instances.length; i > 0; -i--) {
-              if (Drupal.ajax.instances[i-1].ajaxing) {
-                return true;
-              }
-            }
+          if (instance) {
+            return !instance.ajaxing;
+          }
+          else {
+            return true;
           }
-          return false;
         }
 
         return (
@@ -78,8 +77,8 @@
               jQuery(':animated').length === 0
             )
           ) && 
-          Drupal.debounceCount === 0 && 
-          !DrupalIsAjaxing()
+          (typeof Drupal.debounceCount === 'undefined' || Drupal.debounceCount === 0) && 
+          Drupal.ajax.instances.every(IsNotAjaxing)
         );
       }());
 JS;