We use meta_workpace_focus_default_window() to sync the input focus back
to a window after it was on shell UI, this is not really necessary on
Wayland, but it is on X11. What this function does internally is ask
MetaWindowStack about the topmost window and focus+raise that window.
In gnome-shell we set the input focus to the default window every time
the key-focus changes to NULL (see shell-global.c ->
sync_stage_window_focus()). Now when closing the alt-tab switcher and
activating a window while there's an always-on-top window on the
workspace, meta_workspace_focus_default_window() will focus that
always-on-top window right after closing the alt-tab switcher, making it
impossible to focus another window using alt-tab.
To fix this, make meta_workspace_focus_default_window() check if there's
an existing focus_window first, if there is, use that, and if there
isn't, resort to just focusing the topmost one.
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5162
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2328>
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