Fix multitasking view on x and improve comment

Author: leolost2605

background/BackgroundWindow.vala

@@ -132,23 +132,23 @@ public class Gala.Background.BackgroundWindow : Gtk.Window, PantheonWayland.Exte
     }
 
     private void make_background_x11 () {
-        var monitor = (Gdk.X11.Monitor) Gdk.Display.get_default ().get_monitors ().get_item (monitor_index);
-
-        var geom = monitor.geometry;
-
         unowned var xdisplay = ((Gdk.X11.Display) display).get_xdisplay ();
 
-        unowned var x_window = ((Gdk.X11.Surface) get_surface ()).get_xid ();
+        unowned var xwindow = ((Gdk.X11.Surface) get_surface ()).get_xid ();
 
         var atom = xdisplay.intern_atom ("_NET_WM_WINDOW_TYPE", false);
         var dock_atom = xdisplay.intern_atom ("_NET_WM_WINDOW_TYPE_DESKTOP", false);
 
         // (X.Atom) 4 is XA_ATOM
         // 32 is format
         // 0 means replace
-        xdisplay.change_property (x_window, atom, (X.Atom) 4, 32, 0, (uchar[]) dock_atom, 1);
+        xdisplay.change_property (xwindow, atom, (X.Atom) 4, 32, 0, (uchar[]) dock_atom, 1);
+
+        var mutter_prop = xdisplay.intern_atom ("_MUTTER_HINTS", false);
+
+        var mutter_prop_value = "monitor-index=%d".printf (monitor_index);
 
-        xdisplay.move_window (x_window, geom.x, geom.y);
+        xdisplay.change_property (xwindow, mutter_prop, X.XA_STRING, 8, 0, (uchar[]) mutter_prop_value, mutter_prop_value.length);
     }
 
     private static void action_launch (SimpleAction action, Variant? variant) {

src/ShellClients/ShellClientsManager.vala

@@ -229,8 +229,9 @@ public class Gala.ShellClientsManager : Object {
     }
 
     /**
-     * This clone will be valid until Meta.MonitorManager::monitors-changed was emitted.
-     * After that the clone musn't be used. This will only return null if the monitor_index is out of bounds.
+     * This clone will be valid until monitor_index goes out of bounds (i.e. enough monitors were disconnected
+     * so that the number of monitors <= monitor_index). After that the clone musn't be used.
+     * This will only return null if the monitor_index is out of bounds.
      */
     public Clutter.Actor? get_background_clone_for_monitor (int monitor_index) {
         if (monitor_index < 0 || monitor_index > background_windows.length) {
@@ -307,6 +308,15 @@ public class Gala.ShellClientsManager : Object {
                     make_centered (window);
                     break;
 
+                case "monitor-index":
+                    int parsed; // Will be used as Pantheon.Desktop.HideMode which is a 5 value enum so check bounds for that
+                    if (int.try_parse (val, out parsed)) {
+                        make_background (window, parsed);
+                    } else {
+                        warning ("Failed to parse %s as monitor index", val);
+                    }
+                    break;
+
                 default:
                     break;
             }