> Which means that if your element's width is 100vw and the page height is larger than the viewport height so that it shows a vertical scrollbar, then suddenly you get a horizontal scrollbar as well.

Oh wow that's insane.

edit: apparently the "useful" behaviour used to be opted-in using overflow: scroll, but only firefox implemented that, they asked if it was really useful (when nobody else cared) and… it was dropped from the spec.