[appDisplay] port from ShellMenu to PopupMenu

This fixes the style, and also makes it keyboard navigable

https://bugzilla.gnome.org/show_bug.cgi?id=619541

js/ui/workspace.js

@@ -736,6 +736,9 @@ Workspace.prototype = {
      * Draw the user's attention to the given window @metaWindow.
      */
     setHighlightWindow: function (metaWindow) {
+        if (!this._lightbox)
+            return;
+
         let actor;
         if (metaWindow != null) {
             let clone = this.lookupCloneForMetaWindow(metaWindow);