[css-anchor-position] Fix the use of the scroll snapshot in determini…

…ng fallback, and close the edge case of a 0-height abspos considered to be fitting in a negative-height CB. #8059

css-anchor-position-1/Overview.bs

@@ -1773,12 +1773,13 @@ even tho they depend on layout and [=used values=].
 
  1. Let |adjusted styles| be the result of [=applying a position option=] |option| to |el|.
 
- 2. Let |el rect| be the size and position of |el|'s margin box,
- when laid out with |adjusted styles|.
- Let |cb rect| be the size and position of |el|'s [=inset-modified containing block=].
+ 2. Let |cb rect| be the size and position of |el|'s [=inset-modified containing block=].
+ If |cb rect| had to be adjusted to ensure
+ it didn't have a negative size,
+ [=iteration/continue=].
 
- 3. If |el| has a [=snapshotted scroll offset=],
- subtract it from the position of both |el rect| and |cb rect|.
+ 3. Let |el rect| be the size and position of |el|'s margin box,
+ when laid out with |adjusted styles|.
 
  4. If |el rect| is not fully contained within |cb rect|,
  [=iteration/continue=].