The background group is currently allocated taking care of the workarea
x, y offset but not of its width/height and this may lead to building a
wrongly sized workspace view when the workarea size is not matching the
monitor size (like when there are struts set).
So, take care of the difference between the workarea and monitor
absolute end coordinates to allocate the background scaled content box.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1892>
f30fa1adc7