Flexbox model will replace old xul box as the default display model so
this patch adds support for that in a whole bunch of styles.
A lot of style rules are marked as "Fx < 112 compatibility" rules and
those can be removed when 112 hits release.
This opaque background allows us to draw statuspanel behind it.
Without this patch, the statupanel could be shown on tabs
that don't yet have associated <browser> such as when restoring
a tab after startup.
