[css-anchor-position-1] Specify scroll offset snapshot timing #10796 (#13675)

* [css-anchor-position-1] Specify scroll offset snapshot timing #10796

Specify that scroll offsets are snapshotted when asked to run snapshot
post-layout state steps.

Also see HTML PR[1] for when "run snapshot post-layout state steps" are
expected to be run.

[1] whatwg/html#11613

* Apply suggestion from @tabatkins

(just rewrapping to semantic line breaks)

---------

Co-authored-by: Tab Atkins Jr. <jackalmage@gmail.com>

Author: lilles

css-anchor-position-1/Overview.bs

@@ -1075,6 +1075,9 @@ while still allowing as much freedom to anchor to various elements as possible,
 [=anchor positioning=] uses a combination of [=remembered scroll offsets=]
 and [=compensating for scroll=].
 
+The scroll offsets which are used to compute [=remembered scroll offsets=] and [=default scroll shift=] 
+are snapshotted when asked to [=run snapshot post-layout state steps=].
+
 <div class=note>
 	The details here are technical, but the gist is:
 
@@ -1236,10 +1239,6 @@ specifically, the [=default anchor element=]:
 	as if affected by a [=transform=]
 	(before any other transforms).
 
-	Issue: Define the precise timing of the snapshot:
-	updated each frame,
-	before style recalc.
-
 	Issue: Similar to [=remembered scroll offset=],
 	can we pay attention to transforms on the [=default anchor element=]?