Issue #3106417 by Etroid: multiple selections filter breaks when filter is collapsible

src/Plugin/better_exposed_filters/filter/DefaultWidget.php

@@ -2,6 +2,8 @@
 
 namespace Drupal\better_exposed_filters\Plugin\better_exposed_filters\filter;
 
+use Drupal\Core\Form\FormStateInterface;
+
 /**
  * Default widget implementation.
  *
@@ -19,4 +21,24 @@ class DefaultWidget extends FilterWidgetBase {
     return TRUE;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function exposedFormAlter(array &$form, FormStateInterface $form_state) {
+    $field_id = $this->getExposedFilterFieldId();
+    $type = $this->getExposedFilterWidgetType();
+
+    parent::exposedFormAlter($form, $form_state);
+
+    if ($type === 'select') {
+      // Workaround to add support for merging process and pre-render functions
+      // to the render array of an element.
+      // @todo remove once core issue is resolved.
+      // @see https://www.drupal.org/project/drupal/issues/2070131
+      $form[$field_id]['#process'][] = ['\Drupal\Core\Render\Element\Select', 'processSelect'];
+      $form[$field_id]['#process'][] = ['\Drupal\Core\Render\Element\Select', 'processAjaxForm'];
+      $form[$field_id]['#pre_render'][] = ['\Drupal\Core\Render\Element\Select', 'preRenderSelect'];
+    }
+  }
+
 }