only in patch2:
unchanged:
--- a/core/tests/Drupal/Tests/BrowserTestBase.php
+++ b/core/tests/Drupal/Tests/BrowserTestBase.php
@@ -906,9 +906,13 @@ protected function submitForm(array $edit, $submit, $form_html_id = NULL) {
     // Ensure that any changes to variables in the other thread are picked up.
     $this->refreshVariables();
 
-    // Log only for JavascriptTestBase tests because for Goutte we log with
-    // ::getResponseLogHandler.
+    // Actions only for JavascriptTestBase.
     if ($this->htmlOutputEnabled && !($this->getSession()->getDriver() instanceof GoutteDriver)) {
+      // Wait on AJAX requests to finish which might have been triggered on load.
+      $this->assertSession()->assertWaitOnAjaxRequest();
+
+      // Log only for JavascriptTestBase tests because for Goutte we log with
+      // ::getResponseLogHandler.
       $out = $this->getSession()->getPage()->getContent();
       $html_output = 'POST request to: ' . $action .
         '<hr />Ending URL: ' . $this->getSession()->getCurrentUrl();