If a client maps a popup in response to a key-down event, but the
mapping doesn't occur until after the user has already released the same
button, we'd immediately dismiss the popup. This is problematic, as one
often presses and releases a key quite quickly, meaning any popup mapped
on key-down are likely to be dismissed.
Avoid this race condition by accepting serials for key down events, if
the most recent key-up event had the same keycode.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/180
2dbacfa8d6