overviewControls: Sync hover after drag operations

During global grabs, actors miss enter and leave events required
for correct hover tracking. This can cause the workspace switcher
to get stuck while slid out, so ensure the actor's hover state is
synced after drag operations.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/161

js/ui/overviewControls.js

@@ -284,6 +284,11 @@ var ThumbnailsSlider = new Lang.Class({
         return child.get_theme_node().get_length('visible-width');
     },
 
+    _onDragEnd() {
+        this.actor.sync_hover();
+        this.parent();
+    },
+
     _getSlide() {
         if (!this._visible)
             return 0;