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
Some tests were not waiting for the test client to actually issue
destroy commands before checking their effect on the window focus.
Similarly when mutter is supposed to change the focus based on a delay
by sending a WM_TAKE_FOCUS to the client, this also could fail without
synchronization with the client before checking the result.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1325
When requesting to a take-focus window to acquire the input, the client may or
may not respond with a SetInputFocus (this doesn't happen for no-input gtk
windows in fact [to be fixed there too]), in such case we were unsetting the
focus while waiting the reply.
In case the client won't respond, we wait for a small delay (set to 250 ms) for
the take-focus window to grab the input focus before setting it to the default
window.
Added a test for this behavior and for the case in which a window takes the
focus meanwhile we're waiting to focus the default window.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/307
