While c3d13203 ensured that the test-client has actually closed the
window before testing for the focus change, it also made another timing
related issue with the tests more likely to happen. Serveral tests
assert that the focus is set to 'none' after the focussed window has
been closed when the window below does not accept focus. This however
can never be reliably tested, because closing the window triggers
timeout based iteration of a queue of default focus candidate windows.
This starts after the window has been closed and might finish before the
clients have finished synchronizing. This issue is more likely to
trigger the shorter the queue is and the more test clients there are
that could delay the synchronization.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1329
d830d985c1