If we happen to be changing focus to a window *while* taking focus
away from Clutter widgetry, we would unintendedly trigger reentrance
in a way that the old focused window remained in focus, by asking
to focus the default focus window in an untimely manner.
To handle this reentrancy, delay dropping the Clutter key focus
until the window focus changed, so that the focus change will look
up the default focused window in the workspace, and find the up to
date one.
Fixes: ae102ee301 ("x11: Refactor ClutterStage key focus management")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3467>
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