diff --git a/core/includes/pager.inc b/core/includes/pager.inc
index e532a96..2a535bc 100644
--- a/core/includes/pager.inc
+++ b/core/includes/pager.inc
@@ -148,6 +148,10 @@ function pager_default_initialize($total, $limit, $element = 0) {
 function pager_get_query_parameters() {
   $query = &drupal_static(__FUNCTION__);
   if (!isset($query)) {
+    // There is a chance that the pager is rendered in the context of an ajax
+    // form request. Therefore exclude AJAX_FORM_REQUEST from there, as
+    // otherwise clicking the link would be interpreted potentially as form
+    // request, especially when done as POST request using JS.
     $query = UrlHelper::filterQueryParameters(\Drupal::request()->query->all(), array('page', \Drupal\Core\Form\FormBuilderInterface::AJAX_FORM_REQUEST));
   }
   return $query;
diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index 8e6f13a..294846a 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -475,8 +475,7 @@ public function getButtons() {
    * {@inheritdoc}
    */
   public function setCached($cache = TRUE) {
-    // We try to determine whether this is the initial load of the form.
-    // For normal forms it is enough
+    // We determine whether this is the initial rendering of the form.
     if ($this->isMethodType('GET') && !$this->isSubmitted()) {
       throw new \LogicException('Initial form rendering is not allowed to cache the form.');
     }
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index 4ae3a57..239916b 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Core\Render\Element;
 
+use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\PluginBase;
@@ -231,8 +232,7 @@ public static function preRenderAjaxForm($element) {
       $element['#attached']['library'][] = 'core/drupal.ajax';
 
       $settings = $element['#ajax'];
-      // Do not support custom URLs, only allow this method to use it for
-      // internal reasons.
+      // Do not support custom URLs, $settings['url'] is generated below.
       if (isset($settings['url'])) {
         throw new \InvalidArgumentException('Custom AJAX URLs are not allowed');
       }
@@ -249,8 +249,8 @@ public static function preRenderAjaxForm($element) {
         $query = \Drupal::request()->query->all();
         $query[FormBuilderInterface::AJAX_FORM_REQUEST] = TRUE;
 
-        $settings += ['options' => []];
-        $settings['options'] += ['query' => []];
+        // Add options['query'] => [] as default value.
+        $settings = NestedArray::mergeDeep($settings, ['options' => ['query' => []]]);
         $options['query'] = $settings['options']['query'] + $query;
 
         $settings['url'] = Url::fromRoute('<current>', [], $options)->toString();
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
index 3eca4cc..fe73b5f 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
@@ -482,6 +482,7 @@ public function testSetCachedPost() {
    * @covers ::setCached
    *
    * @expectedException \LogicException
+   * @expectedExceptionMessage Initial form rendering is not allowed to cache the form.
    */
   public function testSetCachedGet() {
     $form_state = new FormState();
@@ -498,7 +499,7 @@ public function testSetCachedGetWithSubmission() {
     $form_state->setSubmitted();
     $form_state->setCached();
 
-    // Get form submissions should not be cached.
+    // GET form submissions should not be cached.
     $this->assertFalse($form_state->isCached());
   }