[css-overflow-4] Allow scrollable overflow to be clipped in off-axis

[flackr]

When developers create scrolling containers that are only intended to scroll in one direction, they'll sometimes have content which slightly overflow in the other axis. Per the css-overflow-3 spec:

The visible/clip values of overflow compute to auto/hidden (respectively) if one of overflow-x or overflow-y is neither visible nor clip.

This seems to mean that even if you set the off axis overflow to clip it will still compute to hidden. While this is not a problem for user scrolling, overflow: hidden is still programmatically scrollable. Calls to scrollIntoView or anchor link scrolls can accidentally vertically scroll to your overflowing content. Worse yet, a user has no way to undo this after it happens.

This originally came up as a scroll-marker bug crbug.com/422733086 (Demo), however this problem can easily occur with existing content. E.g. consider this demo with "accidental" vertical scrolling when clicking on an item to scroll it into view: https://jsbin.com/voxohip/edit?html,css,js,output

Proposal: We should allow overflow clip in the off axis to make that axis not scrollable. E.g. overflow: auto clip; should allow creating a scroll container which cannot be scrolled vertically.

[flackr]

@una FYI