diff -u b/core/lib/Drupal/Core/Controller/AjaxController.php b/core/lib/Drupal/Core/Controller/AjaxController.php
--- b/core/lib/Drupal/Core/Controller/AjaxController.php
+++ b/core/lib/Drupal/Core/Controller/AjaxController.php
@@ -48,7 +48,8 @@
     // Remove the accept header so the subrequest does not end up back in this
     // controller.
     $request->headers->remove('accept');
-    // @todo Explain why this is needed.
+    // Remove the header in order to let the subrequest not think that it's an
+    // ajax request, see \Drupal\Core\ContentNegotiation.
     $request->headers->remove('x-requested-with');
 
     $response = $this->container->get('http_kernel')->forward($controller, $attributes->all(), $request->query->all());