[css-scroll-anchoring] [css-scroll-snap] What's the optimal viewing rect on the root scroller? #4393

https://drafts.csswg.org/css-scroll-snap-1/#optimal-viewing-region

Firefox uses the visual viewport. Blink (if my read of their code is correct) uses the layout viewport (I think?).

The spec is unclear. It says "the scrollport".

See https://bugzilla.mozilla.org/show_bug.cgi?id=1585317#c4 for context.

If it should use the visual viewport, then scroll-padding should in theory resolve against that, rather than the layout viewport... right?

cc: @tabatkins @fantasai @bokand @skobes