This looks simpler, a lot smoother on my machine, and shouldn't cause
content resizes. It also doesn't set custom properties on the root which
causes an expensive restyle, specially on big documents (like with many
tabs open).
Maybe eventually we can use view transitions for this, seems like a good
use-case.
We need to move the background to `#sidebar-main` rather than the
wrapper because during the shrinking animation the wrapper is small and
the `#sidebar-main` element would be transparent, which we don't want.
Differential Revision: https://phabricator.services.mozilla.com/D220846
39dbb62d25