diff --git a/core/lib/Drupal/Core/Controller/FormController.php b/core/lib/Drupal/Core/Controller/FormController.php
index 03a76d4..6a9b9d5 100644
--- a/core/lib/Drupal/Core/Controller/FormController.php
+++ b/core/lib/Drupal/Core/Controller/FormController.php
@@ -8,7 +8,6 @@
 namespace Drupal\Core\Controller;
 
 use Drupal\Core\DependencyInjection\DependencySerializationTrait;
-use Drupal\Core\EventSubscriber\FormControllerSubscriber;
 use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Form\FormState;
 use Drupal\Core\Routing\RouteMatchInterface;
@@ -77,12 +76,6 @@ public function getContentResult(Request $request, RouteMatchInterface $route_ma
     unset($args[0], $args[1]);
     $form_state->addBuildInfo('args', array_values($args));
 
-    // If this form is being submitted back to its original URL, disable all
-    // redirects.
-    if ($request->query->get(FormControllerSubscriber::POST_TO_ORIGINAL_URL) === FormControllerSubscriber::DRUPAL_AJAX_POST) {
-      $form_state->disableRedirect();
-    }
-
     return $this->formBuilder->buildForm($form_object, $form_state);
   }
 
diff --git a/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
index cc13ae4..1289596 100644
--- a/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -12,6 +12,7 @@
 use Drupal\Component\Utility\SortArray;
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
 use Symfony\Component\Validator\ConstraintViolationInterface;
 use Symfony\Component\Validator\ConstraintViolationListInterface;
 
@@ -243,6 +244,9 @@ protected function formMultipleElements(FieldItemListInterface $items, array &$f
             'callback' => array(get_class($this), 'addMoreAjax'),
             'wrapper' => $wrapper_id,
             'effect' => 'fade',
+            // @todo Remove this.
+            'url' => Url::fromRoute('system.ajax'),
+            'cache_form' => TRUE,
           ),
         );
       }
diff --git a/core/lib/Drupal/Core/Form/FormAjaxException.php b/core/lib/Drupal/Core/Form/FormAjaxException.php
index 4a405f6..66d4274 100644
--- a/core/lib/Drupal/Core/Form/FormAjaxException.php
+++ b/core/lib/Drupal/Core/Form/FormAjaxException.php
@@ -8,7 +8,7 @@
 namespace Drupal\Core\Form;
 
 /**
- * @todo.
+ * Custom exception to break out of AJAX form processing.
  */
 class FormAjaxException extends \Exception {
 
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index 7c0eba3..8418faf 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -247,7 +247,7 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
 
     // If this form should post to the original URL, disable all form redirects.
     $request = $this->requestStack->getCurrentRequest();
-    if ($post_to_original_url = $request->query->get(FormControllerSubscriber::POST_TO_ORIGINAL_URL) === FormControllerSubscriber::DRUPAL_AJAX_POST) {
+    if ($post_to_original_url = ($request->query->get(FormControllerSubscriber::POST_TO_ORIGINAL_URL) === FormControllerSubscriber::DRUPAL_AJAX_POST)) {
       $form_state->disableRedirect();
     }
 
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index aaf2969..1c788e9 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\Core\Render\Element;
 
-use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\EventSubscriber\FormControllerSubscriber;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\PluginBase;
@@ -130,6 +129,10 @@ public static function preRenderGroup($element) {
    */
   public static function processAjaxForm(&$element, FormStateInterface $form_state, &$complete_form) {
     $element = static::preRenderAjaxForm($element);
+    // @todo Remove this.
+    if (!empty($element['#ajax']['cache_form'])) {
+      $form_state->setCached();
+    }
     return $element;
   }
 
@@ -151,6 +154,7 @@ public static function processAjaxForm(&$element, FormStateInterface $form_state
    *   - #ajax['parameters']
    *   - #ajax['effect']
    *   - #ajax['accepts']
+   *   - #ajax['cache_form']
    *
    * @return array
    *   The processed element with the necessary JavaScript attached to it.
diff --git a/core/modules/file/src/Element/ManagedFile.php b/core/modules/file/src/Element/ManagedFile.php
index 40d0c31..9719ea9 100644
--- a/core/modules/file/src/Element/ManagedFile.php
+++ b/core/modules/file/src/Element/ManagedFile.php
@@ -146,6 +146,8 @@ public static function processManagedFile(&$element, FormStateInterface $form_st
 
     $ajax_settings = [
       'url' => Url::fromRoute('file.ajax_upload'),
+      // @todo Remove this.
+      'cache_form' => TRUE,
       'options' => [
         'query' => [
           'element_parents' => implode('/', $element['#array_parents']),
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 721fdd9..a206f0e 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1504,6 +1504,8 @@ protected function drupalGetAjax($path, array $options = array(), array $headers
    *   is done by drupalPostAjaxForm(). This string is literally appended to the
    *   POST data, so it must already be urlencoded and contain a leading "&"
    *   (e.g., "&extra_var1=hello+world&extra_var2=you%26me").
+   *
+   * @return bool|string|null
    */
   protected function drupalPostForm($path, $edit, $submit, array $options = array(), array $headers = array(), $form_html_id = NULL, $extra_post = NULL) {
     $submit_matches = FALSE;
@@ -2622,7 +2624,8 @@ protected function prepareRequestForGenerator($clean_urls = TRUE, $override_serv
    */
   protected function buildUrl($path, array $options = array()) {
     if ($path instanceof Url) {
-      return $path->setAbsolute()->toString();
+      $absolute_path = clone $path;
+      return $absolute_path->setAbsolute()->toString();
     }
     // The URL generator service is not necessarily available yet; e.g., in
     // interactive installer tests.
diff --git a/core/modules/views_ui/admin.inc b/core/modules/views_ui/admin.inc
index 6c72b03..b5cbda5 100644
--- a/core/modules/views_ui/admin.inc
+++ b/core/modules/views_ui/admin.inc
@@ -53,7 +53,9 @@ function views_ui_add_ajax_trigger(&$wrapping_element, $trigger_key, $refresh_pa
   $triggering_element['#ajax']['callback'] = 'views_ui_ajax_update_form';
 
   // Specify the #ajax URL in order to retain form caching.
+  // @todo Remove this.
   $triggering_element['#ajax']['url'] = Url::fromRoute('system.ajax');
+  $triggering_element['#ajax']['cache_form'] = TRUE;
 
   // We do not use \Drupal\Component\Utility\Html::getUniqueId() to get an ID
   // for the AJAX wrapper, because it remembers IDs across AJAX requests (and