workspaceThumbnail: only update _porthole if the overview is visible

Otherwise it happens that porthole is computed again after that the
overlay is hidden (triggered by a layout reallocation) and thus not
regenerated again afterwards.

https://bugzilla.gnome.org/show_bug.cgi?id=792687

js/ui/workspaceThumbnail.js

@@ -1159,7 +1159,7 @@ var ThumbnailsBox = new Lang.Class({
     // The "porthole" is the portion of the screen that we show in the
     // workspaces
     _ensurePorthole() {
-        if (!Main.layoutManager.primaryMonitor)
+        if (!Main.layoutManager.primaryMonitor || !Main.overview.visible)
             return false;
 
         if (!this._porthole)