We build controls layout using the whole monitor vertical space as
available, however extensions or external apps in X11 may reduce the
workarea size horizontally and the shell should always take care of it.
Given that we're already assuming that the allocation is monitor-based
and that we're adjusting it to the workarea, we can just make it more
explicit by using a workarea box that is used as the allocation area.
As per this, we also apply the same logic of applied to the vertical
dimension to the horizontal one.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1892>
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