clutter/offscreen-effect: Use the paint volume origin as the FBO offset

Before now it was assumed that the only negative local coordinates would
be those introduced by `_clutter_actor_box_enlarge_for_effects`, and we
used the difference for `fbo_offset_x/y`. But that was misguided (of me)
because gnome-shell can give us paint volumes at negative coordinates too,
like when rendering `box-shadow` on the top or left edge of an actor.

The maximum extents of negative coordinates we might need to render are
in fact the (enlarged) left and top edges of the paint volume. So use
those as the FBO offset. This places the actor's local origin correctly
within the FBO and thus also ensures it's not over-clipped at the edges
of the FBO, which now line up with the enlarged extents of the paint
volume.

This fixes one third of
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1090
The other fixes required are !1053 and gnome-shell!1417.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1538>

1 files changed, 5 insertions, 6 deletions

diff --git a/clutter/clutter/clutter-offscreen-effect.c b/clutter/clutter/clutter-offscreen-effect.c
index cfb0ddc95..c211768f7 100644
--- a/clutter/clutter/clutter-offscreen-effect.c
+++ b/clutter/clutter/clutter-offscreen-effect.c
@@ -260,7 +260,7 @@ clutter_offscreen_effect_pre_paint (ClutterEffect       *effect,
   ClutterOffscreenEffect *self = CLUTTER_OFFSCREEN_EFFECT (effect);
   ClutterOffscreenEffectPrivate *priv = self->priv;
   CoglFramebuffer *offscreen;
-  ClutterActorBox raw_box, box;
+  ClutterActorBox box;
   ClutterActor *stage;
   graphene_matrix_t projection, modelview;
   const ClutterPaintVolume *volume;
@@ -297,19 +297,18 @@ clutter_offscreen_effect_pre_paint (ClutterEffect       *effect,
       ClutterPaintVolume mutable_volume;
 
       _clutter_paint_volume_copy_static (volume, &mutable_volume);
-      _clutter_paint_volume_get_bounding_box (&mutable_volume, &raw_box);
+      _clutter_paint_volume_get_bounding_box (&mutable_volume, &box);
       clutter_paint_volume_free (&mutable_volume);
     }
   else
     {
-      clutter_actor_get_allocation_box (priv->actor, &raw_box);
+      clutter_actor_get_allocation_box (priv->actor, &box);
     }
 
-  box = raw_box;
   _clutter_actor_box_enlarge_for_effects (&box);
 
-  priv->fbo_offset_x = box.x1 - raw_box.x1;
-  priv->fbo_offset_y = box.y1 - raw_box.y1;
+  priv->fbo_offset_x = box.x1;
+  priv->fbo_offset_y = box.y1;
 
   clutter_actor_box_scale (&box, ceiled_resource_scale);
   clutter_actor_box_get_size (&box, &target_width, &target_height);