compositor: Keep track of the top window actor on each view

First, add logic in MetaCompositorView to find topmost visible
MetaWindowActor on its view, and expose it through a new API.

Then, queue an update to find the top MetaWindowActor of each
MetaCompositorView in the following cases:
  1. The MetaCompositor is in its initial state.
  2. The window stack order has changed.
  3. A window has changed its visibility.
  4. A "stage-views-changed" signal was emitted for a MetaWindowActor.

Finally, perform the queued update in meta_compositor_before_paint (),
and assert that an update isn't queued during painting. This ensures
that the top window actor in the MetaCompositorView remains up-to-date
and available to child classes of MetaCompositor throughout the entire
paint stage.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2526>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/compositor/meta-window-actor.c b/src/compositor/meta-window-actor.c
index ae1fa4d90..9fd9f758c 100644
--- a/src/compositor/meta-window-actor.c
+++ b/src/compositor/meta-window-actor.c
@@ -53,6 +53,8 @@ typedef struct _MetaWindowActorPrivate
   MetaWindow *window;
   MetaCompositor *compositor;
 
+  gulong stage_views_changed_id;
+
   MetaSurfaceActor *surface;
 
   int geometry_scale;
@@ -403,6 +405,15 @@ init_surface_actor (MetaWindowActor *self)
 }
 
 static void
+on_stage_views_changed (MetaWindowActor *self)
+{
+  MetaWindowActorPrivate *priv =
+    meta_window_actor_get_instance_private (self);
+
+  meta_compositor_window_actor_stage_views_changed (priv->compositor);
+}
+
+static void
 meta_window_actor_constructed (GObject *object)
 {
   MetaWindowActor *self = META_WINDOW_ACTOR (object);
@@ -412,6 +423,12 @@ meta_window_actor_constructed (GObject *object)
 
   priv->compositor = window->display->compositor;
 
+  priv->stage_views_changed_id =
+    g_signal_connect (self,
+                      "stage-views-changed",
+                      G_CALLBACK (on_stage_views_changed),
+                      NULL);
+
   /* Hang our compositor window state off the MetaWindow for fast retrieval */
   meta_window_set_compositor_private (window, object);
 
@@ -445,6 +462,8 @@ meta_window_actor_dispose (GObject *object)
 
   priv->disposed = TRUE;
 
+  g_clear_signal_handler (&priv->stage_views_changed_id, self);
+
   meta_compositor_remove_window_actor (compositor, self);
 
   g_clear_object (&priv->window);