[css-anchor-position] Fix the use of the scroll snapshot in determining fallback, and close the edge case of a 0-height abspos considered to be fitting in a negative-height CB. #8059

Author: tabatkins

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=].