panel: Update the hot corner when the panel box allocation changes

This fixes the missing hotcorner in the overview.

https://bugzilla.gnome.org/show_bug.cgi?id=689537

js/ui/panel.js

@@ -655,6 +655,13 @@ const ActivitiesButton = new Lang.Class({
         }));
 
         this._xdndTimeOut = 0;
+
+        // Since the hot corner uses stage coordinates, Clutter won't
+        // queue relayouts for us when the panel moves. Queue a relayout
+        // when that happens.
+        Main.layoutManager.connect('panel-box-changed', Lang.bind(this, function() {
+            container.queue_relayout();
+        }));
     },
 
     _containerGetPreferredWidth: function(actor, forHeight, alloc) {