Issue #2361383 by pivica: Fixed Drupal modal dialog should use ui-front class.

core/misc/dialog/dialog.ajax.js

@@ -15,7 +15,10 @@
       // dialogs. Non-modal dialogs are responsible for creating their own
       // elements, since there can be multiple non-modal dialogs at a time.
       if (!$('#drupal-modal').length) {
-        $('<div id="drupal-modal" />').hide().appendTo('body');
+        // Add 'ui-front' jQuery UI class so jQuery UI widgets like autocomplete
+        // sit on top of dialogs. For more information see
+        // http://api.jqueryui.com/theming/stacking-elements/.
+        $('<div id="drupal-modal" class="ui-front"/>').hide().appendTo('body');
       }
 
       // Special behaviors specific when attaching content within a dialog.