[mutter] clutter/actor: Don't allocate actors if only the absolute origin changed

[Jonas Ådahl <jadahl src gnome org>]

commit 720360b07ad06255dad39643397118568ec656fa
Author: Jonas Dreßler <verdre v0yd nl>
Date:   Mon May 11 15:11:07 2020 +0200

    clutter/actor: Don't allocate actors if only the absolute origin changed
    
    For actors which don't have needs_allocation set to TRUE and where the
    new allocation wouldn't be different from the old one, the allocate()
    vfunc doesn't have to be called. We still did this in case a parent
    actor was moved though (so the absolute origin changed), because we
    needed to propagate the ABSOLUTE_ORIGIN_CHANGED allocation flag down to
    all actors.
    
    Since that flag is now removed and got replaced with a private property,
    we can simply notify the children about the absolute allocation change
    using the existing infrastructure and safely stop allocating children at
    this point.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1247

 clutter/clutter/clutter-actor.c | 35 ++++++++++++++++++-----------------
 1 file changed, 18 insertions(+), 17 deletions(-)
---
diff --git a/clutter/clutter/clutter-actor.c b/clutter/clutter/clutter-actor.c
index c0eb4a134f..f711038048 100644
--- a/clutter/clutter/clutter-actor.c
+++ b/clutter/clutter/clutter-actor.c
@@ -10115,7 +10115,6 @@ clutter_actor_allocate (ClutterActor          *self,
 {
   ClutterActorBox old_allocation, real_allocation;
   gboolean origin_changed, size_changed;
-  gboolean stage_allocation_changed;
   ClutterActorPrivate *priv;
 
   g_return_if_fail (CLUTTER_IS_ACTOR (self));
@@ -10178,23 +10177,25 @@ clutter_actor_allocate (ClutterActor          *self,
   size_changed = (real_allocation.x2 != old_allocation.x2 ||
                   real_allocation.y2 != old_allocation.y2);
 
-  stage_allocation_changed =
-    priv->absolute_origin_changed || origin_changed || size_changed;
-
-  /* If we get an allocation "out of the blue"
-   * (we did not queue relayout), then we want to
-   * ignore it. But if we have needs_allocation set,
-   * we want to guarantee that allocate() virtual
-   * method is always called, i.e. that queue_relayout()
-   * always results in an allocate() invocation on
-   * an actor.
+  /* When needs_allocation is set but we didn't move nor resize, we still
+   * want to call the allocate() vfunc because a child probably called
+   * queue_relayout() and needs a new allocation.
    *
-   * The optimization here is to avoid re-allocating
-   * actors that did not queue relayout and were
-   * not moved.
+   * In case needs_allocation isn't set and we didn't move nor resize, we
+   * can safely stop allocating, but we need to notify the sub-tree in case
+   * our absolute origin changed.
    */
-  if (!priv->needs_allocation && !stage_allocation_changed)
+  if (!priv->needs_allocation && !origin_changed && !size_changed)
     {
+      if (priv->absolute_origin_changed)
+        {
+          _clutter_actor_traverse (self,
+                                   CLUTTER_ACTOR_TRAVERSE_DEPTH_FIRST,
+                                   absolute_allocation_changed_cb,
+                                   NULL,
+                                   NULL);
+        }
+
       CLUTTER_NOTE (LAYOUT, "No allocation needed");
       goto out;
     }
@@ -10202,10 +10203,10 @@ clutter_actor_allocate (ClutterActor          *self,
   if (CLUTTER_ACTOR_IS_MAPPED (self))
     self->priv->needs_paint_volume_update = TRUE;
 
-  if (!stage_allocation_changed)
+  if (!origin_changed && !size_changed)
     {
       /* If the actor didn't move but needs_allocation is set, we just
-       * need to allocate the children */
+       * need to allocate the children (see comment above) */
       clutter_actor_allocate_internal (self, &real_allocation);
       goto out;
     }