[css-scroll-snap] making proximity snapping more customizable

It was suggested to me that it will be useful to make proximity more configurable.
At the moment the interpretation of proximity is left to the UA (See here). For example Chrome implementation is simple that it snaps if the snap area is within a fixed distance of the appropriate snap alignment (30% of scroller size in that axis).

I think we should consider providing customizability for scroll snap to the developers. This would be valuable for some type of content where it is desirable to snap only if the content is very close to the edge of the scroller.

API Ideas

Here are few APIs ideas that we can explore:

1. Make proximity a css function e.g., scroll-snap-type: proximity(50px) or even allow a margin like 2/4-value syntax scroll-snap-type: proximity(10% 20%)

2. Introduce a new property scroll-snap-proximity-hint: 50px. This can be done either on snap area or container (or both?)

Some interesting discussions to have:

• Do we need to have this control per snap area or having it at the scrollcontainer is enough? At the moment, I think having it per container is sufficient but happy to re-consider.
• Should this be a hint to UA algorithm or do we want a more precise definition? I am curious if other UA engines are doing anything more sophisticated for proximity calculation. If all engines are doing a simple distance check perhaps we should specify this which has the advantage of interoperability and predictability for authors.

Start with the CSS Scroll Snap specification section linked in the issue, then review the proposed proximity function and new property ideas. Compare how user agents currently determine proximity, as suggested by the issue, and document a resolved API and behavior proposal before attempting an implementation.