Issue #2677414 by blue_waters: VerticalTabs' #default_tab broken due to...

Issue #2677414 by blue_waters: VerticalTabs' #default_tab broken due to wrong/mismatching class name

core/lib/Drupal/Core/Render/Element/VerticalTabs.php

@@ -136,7 +136,7 @@ public static function processVerticalTabs(&$element, FormStateInterface $form_s
     $element[$name . '__active_tab'] = array(
       '#type' => 'hidden',
       '#default_value' => $element['#default_tab'],
-      '#attributes' => array('class' => array('vertical-tabs-active-tab')),
+      '#attributes' => array('class' => array('vertical-tabs__active-tab')),
     );
     // Clean up the active tab value so it's not accidentally stored in
     // settings forms.