[Workspace] Disconnect reposition handler when leaving overview

As the repositioning after closing a window preview is delayed, it is
possible that the timeout is triggered while leaving the overview. In
that case the previews move to the new overview position and are changed
abruptly to the original window position when the overview zoom is done.
To prevent this, disconnect the handler when leaving the overview.

js/ui/workspace.js

@@ -1366,6 +1366,10 @@ Workspace.prototype = {
 
         this._hideAllOverlays();
 
+        if (this._repositionWindowsId > 0) {
+            Mainloop.source_remove(this._repositionWindowsId);
+            this._repositionWindowsId = 0;
+        }
         Main.overview.connect('hidden', Lang.bind(this,
                                                   this._doneLeavingOverview));