If a write_async() comes up while we are flushing on the background,
the task will be queued, but not deemed a reason on itself to keep
flushing (and finish the task) after a property delete event.
To fix this, do not ever queue up write_async tasks (this leaves
priv->pending_task only used for flush(), so the "flush to end"
behavior in the background is consistent). We only start a
background flush if there's reasons to do it, but the tasks are
immediately finished.
All data will still be ensured to be transfered on flush/close,
this makes the caller in this situation still able to reach to it.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1198
655a783891