With the introduction of the shallow relayout mechanism another small
but severe regression sneaked into our layout machinery: We might
allocate an actor twice during the same allocation cycle, with one
allocation happening using the wrong parent.
This issue happens when reparenting an actor from a NO_LAYOUT parent to
a non-NO_LAYOUT parent, in particular it triggered a bug in gnome-shell
when DND reparents a child from the NO_LAYOUT uiGroup to the overviews
Workspace actor after a drag ended. The reason the issue happens is the
following chain of events:
1. child of a NO_LAYOUT parent queues a relayout, this child is added to
the priv->pending_relayouts list maintained by ClutterStage
2. child is reparented to a different parent which doesn't have the
NO_LAYOUT flag set, another relayout is queued, this time a different
actor is added to the priv->pending_relayouts list
3. the relayout happens and we go through the pending_relayouts list
backwards, that means the correct relayout queued during 2. happens
first, then the old one happens and we simply call
clutter_actor_allocate_preferred_size() on the actor, that allocation
overrides the other, correct one.
So fix that issue by adding a method to ClutterStage which removes
actors from the pending_relayouts list again and call this method as
soon as an actor with a NO_LAYOUT parent is detached from the stage.
With that in place, we can also remove the check whether an actor is
still on stage while looping through pending_relayouts. In case
something else is going wrong and the actor is not on stage,
clutter_actor_allocate() will warn anyway.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1356
ae83a61e67