This fixes the snap layouts feature on Windows 11.
Instead of using a content attribute (which is somewhat expensive to
look up) use the default appearance and do this where we deal with other
appearance hacks (before building themed backgrounds).
Consolidate this inside a DealWithWindowsAppearanceHacks function along
with the glass stuff.
Differential Revision: https://phabricator.services.mozilla.com/D162757
098175cf10