Makes use of the new buttons sections feature used by the asset shelf to
draw a background behind buttons, but the reset of the region fully
transparent. The UI team agreed on this as a way to minimize dead space,
but also as a better way to communicate our click through feature (where
dead space in overlapping regions forwards events to the underlying
region). The region background theme setting only affects the
non-transparent part of the region, the rest is always fully
transparent. With region overlap disabled, the bar is fully opaque still.
Also see #112241.
55eaa755e3