[css-scroll-snap-1] Drop 'scroll-padding: auto'?

[fantasai]

It's not clear to me that implementations are doing anything particularly interesting with the initial value of scroll-padding, auto. Certainly it's not automatically avoiding sticky headers...

Should we drop this value and set the initial value to 0? And maybe consider adding it back in L2 as a non-initial value that does something useful and automatic?

FWIW, the initial value had two purposes:

• Reflect the amount of default overlap between paging operations / default spacing around scroll-into-view operations so that authors could then control it explicitly by setting a different value. (If we want this, maybe we can call it normal.)
• Try to heuristically account for fixed headers/footers that obscure the content.

[fantasai]

Quick sketch of a herustic:

From each edge of the scrollport establish the current "obscured edge" (initially the scrollport edge). Tracking from the middle of that edge towards the center, recursively check if there is a fixed-positioned box within 1em from the current "obsucred edge"; if so, extend the "obscured edge" to its far border edge.

For the contents of a containing block with sticky boxes, the "obscured edge" is the furthest of the scrollport's global "obscured edge" or the farthest border edge of a sticky-positioned box stuck to that side (as tracked from the center).