window-list: Don't shift message tray when on different monitors

The message tray is moved to the bottom-most monitor when there are
monitors below the primary monitor; in that case, we need to leave
the tray alone.

https://bugzilla.gnome.org/show_bug.cgi?id=728288

extensions/window-list/extension.js

@@ -917,7 +917,8 @@ const WindowList = new Lang.Class({
     },
 
     _updateMessageTrayAnchor: function() {
-        let anchorY = this.actor.visible ? this.actor.height : 0;
+        let sameMonitor = Main.layoutManager.primaryIndex == Main.layoutManager.bottomIndex;
+        let anchorY = this.actor.visible && sameMonitor ? this.actor.height : 0;
 
         Main.messageTray.actor.anchor_y = anchorY;
         Main.messageTray._notificationWidget.anchor_y = -anchorY;