compositor: Don't emit size-changed when only position changes

Waking up gnome-shell and triggering JavaScript listeners of
`size-changed` every time a window was only moved was wasting a lot
of CPU.

This cuts the CPU requirement for dragging windows by around 22%.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/568

src/compositor/compositor.c

@@ -1134,7 +1134,11 @@ meta_compositor_sync_window_geometry (MetaCompositor *compositor,
                                       gboolean        did_placement)
 {
   MetaWindowActor *window_actor = META_WINDOW_ACTOR (meta_window_get_compositor_private (window));
-  meta_window_actor_sync_actor_geometry (window_actor, did_placement);
+  MetaWindowActorChanges changes;
+
+  changes = meta_window_actor_sync_actor_geometry (window_actor, did_placement);
+
+  if (changes & META_WINDOW_ACTOR_CHANGE_SIZE)
     meta_plugin_manager_event_size_changed (compositor->plugin_mgr, window_actor);
 }
 

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

@@ -28,6 +28,12 @@ struct _MetaWindowActorClass
   void (*queue_destroy) (MetaWindowActor *actor);
 };
 
+typedef enum
+{
+  META_WINDOW_ACTOR_CHANGE_SIZE     = 1 << 0,
+  META_WINDOW_ACTOR_CHANGE_POSITION = 1 << 1
+} MetaWindowActorChanges;
+
 void meta_window_actor_queue_destroy   (MetaWindowActor *self);
 
 void meta_window_actor_show (MetaWindowActor *self,
@@ -59,8 +65,10 @@ void     meta_window_actor_set_unredirected    (MetaWindowActor *self,
                                                 gboolean         unredirected);
 
 gboolean meta_window_actor_effect_in_progress  (MetaWindowActor *self);
-void     meta_window_actor_sync_actor_geometry (MetaWindowActor *self,
+
+MetaWindowActorChanges meta_window_actor_sync_actor_geometry (MetaWindowActor *self,
                                                               gboolean         did_placement);
+
 void     meta_window_actor_update_shape        (MetaWindowActor *self);
 void     meta_window_actor_update_opacity      (MetaWindowActor *self);
 void     meta_window_actor_mapped              (MetaWindowActor *self);

src/compositor/meta-window-actor.c

@@ -1120,13 +1120,15 @@ meta_window_actor_queue_destroy (MetaWindowActor *self)
     clutter_actor_destroy (CLUTTER_ACTOR (self));
 }
 
-void
+MetaWindowActorChanges
 meta_window_actor_sync_actor_geometry (MetaWindowActor *self,
                                        gboolean         did_placement)
 {
   MetaWindowActorPrivate *priv =
     meta_window_actor_get_instance_private (self);
   MetaRectangle window_rect;
+  ClutterActor *actor = CLUTTER_ACTOR (self);
+  MetaWindowActorChanges changes = 0;
 
   meta_window_get_buffer_rect (priv->window, &window_rect);
 
@@ -1144,15 +1146,42 @@ meta_window_actor_sync_actor_geometry (MetaWindowActor *self,
    * updates.
    */
   if (is_frozen (self) && !did_placement)
-    return;
+    return META_WINDOW_ACTOR_CHANGE_POSITION | META_WINDOW_ACTOR_CHANGE_SIZE;
 
   if (meta_window_actor_effect_in_progress (self))
-    return;
+    return META_WINDOW_ACTOR_CHANGE_POSITION | META_WINDOW_ACTOR_CHANGE_SIZE;
 
-  clutter_actor_set_position (CLUTTER_ACTOR (self),
+  if (clutter_actor_has_allocation (actor))
-                              window_rect.x, window_rect.y);
+    {
-  clutter_actor_set_size (CLUTTER_ACTOR (self),
+      ClutterActorBox box;
-                          window_rect.width, window_rect.height);
+      float old_x, old_y;
+      float old_width, old_height;
+
+      clutter_actor_get_allocation_box (actor, &box);
+
+      old_x = box.x1;
+      old_y = box.y1;
+      old_width = box.x2 - box.x1;
+      old_height = box.y2 - box.y1;
+
+      if (old_x != window_rect.x || old_y != window_rect.y)
+        changes |= META_WINDOW_ACTOR_CHANGE_POSITION;
+
+      if (old_width != window_rect.width || old_height != window_rect.height)
+        changes |= META_WINDOW_ACTOR_CHANGE_SIZE;
+    }
+  else
+    {
+      changes = META_WINDOW_ACTOR_CHANGE_POSITION | META_WINDOW_ACTOR_CHANGE_SIZE;
+    }
+
+  if (changes & META_WINDOW_ACTOR_CHANGE_POSITION)
+    clutter_actor_set_position (actor, window_rect.x, window_rect.y);
+
+  if (changes & META_WINDOW_ACTOR_CHANGE_SIZE)
+    clutter_actor_set_size (actor, window_rect.width, window_rect.height);
+
+  return changes;
 }
 
 void