https://drafts.csswg.org/css-overflow-3/#overflow-propagation
If clip is applied to the viewport, it must be interpreted as hidden.
However, I wonder if we should revisit this because of the same arguments from #12289. That is, the author may want to disable scrolling, but if there is some content slightly overflowing, then calls to scrollIntoView or anchor link scrolls can accidentally scroll to the overflowing content. Worse yet, a user has no way to undo this after it happens.