[css-scroll-snap] define user scrolling at the edge of a snap area larger than the viewport

[nmoucht]

• This request is related to this interop 2022 test: https://wpt.fyi/results/css/css-scroll-snap/input/snap-area-overflow-boundary.html. In the test, it expects that scrolls past the edge of a currently scrolled snap area don't result in snapping to the next snap position, but instead snap to the end of the snap area. While this is not defined anywhere in the spec (and no browser has this behavior) the current behavior is a poor user experience, as any scroll 1 pixel beyond the border of such a snap area results in a snap to the next snap area, making it difficult to fully scroll to the end of a snap area larger than the viewport. The spec should be updated so that user scrolls near the edge of a snap area larger than the viewport result in snapping to that edge, rather than snapping to the next snap area.

• https://www.w3.org/TR/css-scroll-snap-1/#snap-overflow