diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index d94b9109b6..028396c909 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -239,7 +239,7 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
     // should be initialized to empty rather than to ['destination' => 'foo'].
     $input = $form_state->getUserInput();
     if (!isset($input)) {
-      $input = $form_state->isMethodType('get') ? $request->query->all() : $request->request->all();
+      $input = $request->isMethod('get') ? $request->query->all() : $request->request->all();
       $form_state->setUserInput($input);
     }
 
@@ -560,6 +560,16 @@ public function processForm($form_id, &$form, FormStateInterface &$form_state) {
     $unprocessed_form = $form;
     $form = $this->doBuildForm($form_id, $form, $form_state);
 
+    // Allow an ajax callback while the form is operating in GET mode.
+    // For example when using HOOK_form_views_exposed_form_alter.
+    if ($form_state->isMethodType('get')) {
+      $triggered = $this->requestStack->getCurrentRequest()->request->get('_triggering_element_name');
+      if ($triggered && isset($form[$triggered]) && isset($form[$triggered]['#ajax'])) {
+        $form_state->setTriggeringElement($form[$triggered]);
+        throw new FormAjaxException($form, $form_state);
+      }
+    }
+
     // Only process the input if we have a correct form submission.
     if ($form_state->isProcessingInput()) {
       // Form values for programmed form submissions typically do not include a