Its parent `lockDialogGroup` has exactly the same background-color
set in the same file. Giving them both the same wasted render time on
overdraw, and caused multi-layer blending artifacts (slight flickering
of the grey background) when the login dialog fades in/out.
While the flicker can also be fixed using `set_offscreen_redirect`,
there's no point adding that overhead when the extra layer doesn't need
to be painted. Removing it halves the average render time of the login
animation.
Closes: https://bugs.launchpad.net/bugs/2036388
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3207>
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