[css-anchor-position-1] Remembered Scroll Offset + Scrollable Overflow

[dshin-moz]

Scroll + remembered scroll offset interact in WPT anchor-scroll-chained-002.html.
I believe the reference case is expecting the following:

• JS-induced scroll happens
• Then, the anchor recalculation point is hit
• Then, scrollable overflow for #scroller1 is computed.

This results in a smaller scroller1.scrollHeight, than say, inserting a rAF before the existing JS.

See attached test case, modified for more visibility. asc2.html EDIT: New testcase, outputting to console, as the previous one accidentally caused reflows 🤦 asc2.html)

Testcase aside, could this mean different scrollbar sizes depending on loading time of a website?

[fantasai]

After layout has been performed for abspos, it is additionally shifted by the default scroll shift, as if affected by a transform (before any other transforms).

Transforms affect the scrollable overflow area.

[dshin-moz]

After layout has been performed for abspos, it is additionally shifted by the default scroll shift, as if affected by a transform (before any other transforms).

Transforms affect the scrollable overflow area.

So something like this: positioned-scroll-emulate-transform.html where scrollHeight changes as the "positioned element" scrolls around, right?

OTOH, it's my understanding that positioned elements are laid out at the remembered scroll offset position, as per my read of this:

All anchor references are calculated as if all scroll containers were at their initial scroll position, and then have their associated remembered scroll offset added to them.

Yes, transform-affected scroll area should let people scroll to the positioned element, but there may be a differing portion of scroll overflows contributed by:

• The positioned element's layout position, based on the remembered scroll offset. this will not change until a reflow.
• Scrollable overflow based on "as if by transform." This will go away as the scrollers move and positioned elements no longer overflow.