window-actor: Add API to get the topmost surface actor

On Wayland a window actor may have more than one surface actor,
most importantly when subsurfaces are used.
Add a new function to request the one which is at the top -
it will be used in the next commit.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2211>

src/compositor/meta-window-actor-private.h

@@ -17,6 +17,8 @@ struct _MetaWindowActorClass
                           ClutterFrameInfo *frame_info,
                           int64_t           presentation_time);
 
+  MetaSurfaceActor * (*get_topmost_surface) (MetaWindowActor *actor);
+
   void (*assign_surface_actor) (MetaWindowActor  *actor,
                                 MetaSurfaceActor *surface_actor);
 
@@ -80,6 +82,8 @@ void meta_window_actor_effect_completed (MetaWindowActor  *actor,
 
 MetaSurfaceActor *meta_window_actor_get_surface (MetaWindowActor *self);
 
+MetaSurfaceActor * meta_window_actor_get_topmost_surface (MetaWindowActor *self);
+
 void meta_window_actor_assign_surface_actor (MetaWindowActor  *self,
                                              MetaSurfaceActor *surface_actor);
 

src/compositor/meta-window-actor-wayland.c

@@ -88,6 +88,22 @@ meta_window_actor_wayland_rebuild_surface_tree (MetaWindowActor *actor)
                    &traverse_data);
 }
 
+static MetaSurfaceActor *
+meta_window_actor_wayland_get_topmost_surface (MetaWindowActor *actor)
+{
+  ClutterActor *child_actor;
+
+  for (child_actor = clutter_actor_get_last_child (CLUTTER_ACTOR (actor));
+       child_actor;
+       child_actor = clutter_actor_get_previous_sibling (child_actor))
+    {
+      if (META_IS_SURFACE_ACTOR_WAYLAND (child_actor))
+        return META_SURFACE_ACTOR (child_actor);
+    }
+
+  return NULL;
+}
+
 static void
 meta_window_actor_wayland_assign_surface_actor (MetaWindowActor  *actor,
                                                 MetaSurfaceActor *surface_actor)
@@ -177,6 +193,7 @@ meta_window_actor_wayland_class_init (MetaWindowActorWaylandClass *klass)
   MetaWindowActorClass *window_actor_class = META_WINDOW_ACTOR_CLASS (klass);
   GObjectClass *object_class = G_OBJECT_CLASS (klass);
 
+  window_actor_class->get_topmost_surface = meta_window_actor_wayland_get_topmost_surface;
   window_actor_class->assign_surface_actor = meta_window_actor_wayland_assign_surface_actor;
   window_actor_class->frame_complete = meta_window_actor_wayland_frame_complete;
   window_actor_class->queue_frame_drawn = meta_window_actor_wayland_queue_frame_drawn;

src/compositor/meta-window-actor.c

@@ -107,6 +107,8 @@ static void meta_window_actor_get_property (GObject      *object,
                                             GValue       *value,
                                             GParamSpec   *pspec);
 
+static MetaSurfaceActor * meta_window_actor_real_get_topmost_surface (MetaWindowActor *self);
+
 static void meta_window_actor_real_assign_surface_actor (MetaWindowActor  *self,
                                                          MetaSurfaceActor *surface_actor);
 
@@ -130,6 +132,7 @@ meta_window_actor_class_init (MetaWindowActorClass *klass)
   object_class->get_property = meta_window_actor_get_property;
   object_class->constructed  = meta_window_actor_constructed;
 
+  klass->get_topmost_surface = meta_window_actor_real_get_topmost_surface;
   klass->assign_surface_actor = meta_window_actor_real_assign_surface_actor;
 
   /**
@@ -556,6 +559,18 @@ meta_window_actor_get_surface (MetaWindowActor *self)
   return priv->surface;
 }
 
+static MetaSurfaceActor *
+meta_window_actor_real_get_topmost_surface (MetaWindowActor *self)
+{
+  return meta_window_actor_get_surface (self);
+}
+
+MetaSurfaceActor *
+meta_window_actor_get_topmost_surface (MetaWindowActor *self)
+{
+  return META_WINDOW_ACTOR_GET_CLASS (self)->get_topmost_surface (self);
+}
+
 /**
  * meta_window_actor_is_destroyed:
  * @self: a #MetaWindowActor