compositor.c (x11): Sink the background actors to the bottom of

the window_group when adding them.

Prior to 3f76365 (wlr-layer-shell support) we created background
actors once at Cinnamon startup, and they never needed restacking.

Now their creation is deferred until the monitor manager is
initialized, which may be after other windows have been added to
the group (like nemo-desktop), so we need to make sure they get
sent to the bottom.

Author: mtwebster

src/compositor/compositor.c

@@ -607,6 +607,13 @@ rebuild_x11_background_actors (MetaCompositor *compositor)
       clutter_actor_set_size (actor, rect.width, rect.height);
 
       clutter_actor_add_child (priv->window_group, actor);
+
+      /* add_child() stacks the actor at the top of the window group.  On a
+       * monitors-changed rebuild that leaves the new background above the
+       * desktop windows (bottom_window_group) until the next restack.
+       * Sink it to the bottom where backgrounds belong. */
+      clutter_actor_set_child_below_sibling (priv->window_group, actor, NULL);
+
       priv->background_actors = g_list_append (priv->background_actors, actor);
     }
 }