[cssom-view] Clarify scrollTop/scrollLeft behavior for ongoing smooth scrolls.

https://github.com/w3c/csswg-drafts/blob/16331cc5307a9f929117ed4b81a1d62207870505/cssom-view-1/Overview.bs#L371

About that line specifically:

Abort any ongoing smooth scroll for box.

Makes sense when setting a 2D position through scrollTo & scrollIntoView, but not as much when setting scrollLeft/Top properties, which most people (and browsers today) would assume to be independent properties.

Specifically, should these two blocks of code behave differently when smooth-scrolling is on than if it isn't:

    element.scrollLeft = x;
    element.scrollTop = y;

    element.scrollTop = y;
    element.scrollLeft = x;

It sounds reasonable that both of these should end up [smooth] scrolling to (x,y), but some interpretations of the spec could lead a browser to implement something that would smooth scroll to (0,y) or (x,0) respectively (assuming origin at (0,0)).

Reference mozilla bug where there are questions on the spec: https://bugzilla.mozilla.org/show_bug.cgi?id=1671283

Start with cssom-view-1/Overview.bs at the linked line around “Abort any ongoing smooth scroll for box,” then compare the two scrollTop/scrollLeft assignment sequences described in the issue. Review Mozilla bug 1671283 for the implementation questions; done means the specification clearly defines whether each sequence preserves the other coordinate during smooth scrolling.