[css-scroll-anchoring-1] Anchor adjustment only happens in block flow direction

[eqrion]

Scroll adjustments are defined as only happening in the block flow direction.

What's the reasoning behind this? It'd be good to include it in the specification, as it's come up during implementation review.

[eqrion]

cc @dbaron @theres-waldo @skobes

[jonjohnjohnson]

@eqrion Might be nothing, but while you're combing through the spec to implement scroll-anchoring in gecko, it may be worth checking into and ensuring things jive well with the current language in https://drafts.csswg.org/css-align/#overflow-scroll-position (#1425).

[skobes-chromium]

I think we encountered a page with an unexpected horizontal scroll adjustment, so decided to err on the side of being conservative here.