clutter/stage: Turn get_key_focus() into a proper getter

The method currently returns the stage itself when the property
is NULL.

This has become particularly problematic as the method is detected
as getter by gobject introspection, and gjs now optimizes property
accesses by calling the getter method instead.

Address this by turning the method into a genuine getter without
falling back to the stage.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4256>

clutter/clutter/clutter-stage.c

@@ -2171,7 +2171,7 @@ clutter_stage_set_key_focus (ClutterStage *stage,
  *
  * Retrieves the actor that is currently under key focus.
  *
- * Return value: (transfer none): the actor with key focus, or the stage
+ * Return value: (transfer none) (nullable): the actor with key focus
  */
 ClutterActor *
 clutter_stage_get_key_focus (ClutterStage *stage)
@@ -2181,10 +2181,7 @@ clutter_stage_get_key_focus (ClutterStage *stage)
   g_return_val_if_fail (CLUTTER_IS_STAGE (stage), NULL);
 
   priv = clutter_stage_get_instance_private (stage);
-  if (priv->key_focused_actor)
-    return priv->key_focused_actor;
-
-  return CLUTTER_ACTOR (stage);
+  return priv->key_focused_actor;
 }
 
 /*** Perspective boxed type ******/

src/backends/meta-stage.c

@@ -307,12 +307,6 @@ key_focus_actor_changed (ClutterStage *stage,
 {
   ClutterActor *key_focus = clutter_stage_get_key_focus (stage);
 
-  /* If there's no explicit key focus, clutter_stage_get_key_focus()
-   * returns the stage.
-   */
-  if (key_focus == CLUTTER_ACTOR (stage))
-    key_focus = NULL;
-
   clutter_stage_set_active (stage, key_focus != NULL);
 }
 

src/core/events.c

@@ -77,7 +77,7 @@ stage_has_key_focus (MetaDisplay *display)
 {
   ClutterStage *stage = stage_from_display (display);
 
-  return clutter_stage_get_key_focus (stage) == CLUTTER_ACTOR (stage);
+  return clutter_stage_get_key_focus (stage) == NULL;
 }
 
 static gboolean

src/x11/meta-x11-display.c

@@ -1301,7 +1301,7 @@ stage_has_focus_actor (MetaX11Display *x11_display)
 
   key_focus = clutter_stage_get_key_focus (stage);
 
-  return key_focus != CLUTTER_ACTOR (stage);
+  return key_focus != NULL;
 }
 
 static void