Instead of using overflow:hidden on the body to hide the scrollbar, this
patch sets scrollbar-width:none on the html element. In some cases
overflow:hidden is set on non-root scrollers (i.e. div elements); in those
cases it is replaced by overflow:scroll;scrollbar-width:none to get an
equivalent effect.
One test had a pre-existing visible scrollbar on a nested scrollframe, but
which started failing with a small fuzz difference. I left the scrollbar as-is
and added an annotation to the reftest.list file.
Note that this only updates the tests that use reftest-async-scroll as those
were the cases that were easily detectable, and causing problems with the
apz.allow_zooming=true pref.
Differential Revision: https://phabricator.services.mozilla.com/D82032
3bc44b4a60