[css-overscroll] overscroll-behavior should not prevent visual viewport "panning"

[vmpstr]

If I have a scroller with overscroll-behavior: contain, and I pinch-zoom in (on my macbook), then while my mouse cursor is over this contained scroller, I am unable to pan around the visual viewport.

This is probably a correct behavior per spec, since the visual viewport refers to scrolling and overscroll-behavior contains the scrolling. This is also reproducible in Safari, Firefox, and Chrome. However, as a user, I find this to be not what I would expect.

[flackr]

I agree. It's a bit unfortunate that we called viewport panning scrolling (exposed via scroll events and scroll offsets on the visual viewport api) as just from the terms it sounds like it should be prevented. However from a user perspective, even if I reach the limit of a scroll container and that scroll container doesn't have any more scrolling available I shouldn't be stuck having to pinch to move the visual viewport.

I propose that once we encounter an overscroll-behavior: contain | none element, we immediately jump to panning the visual viewport.

We should still prevent browser overscroll gestures such as back navigation and pull to refresh, as this is a core use case for this API to avoid accidental back navigations in horizontal scrolling experiences.