The 'safe' flag actually causes overflow to be lost, in cases where the
flex container is set up to scroll in the reverse direction. The spec says
to use that flag here, but I don't think other browsers do (and we didn't
either until recently); and I'm proposing we change the spec in
https://github.com/w3c/csswg-drafts/issues/11937
Differential Revision: https://phabricator.services.mozilla.com/D241670
94e23b3d89