[css-anchor-position] Redo the previous change, while also reversing the nonsensical breakage introduced in ab02227. #8059

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1773,14 +1773,25 @@ 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 |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. Let |el rect| be the size and position of |el|'s margin box,
+		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=].
+
+		3. If |el| has a [=snapshotted scroll offset=]:
+
+			1. Set |el rect| to the result of shifting it
+				by the [=snapshotted scroll offset=].
+
+			2. Recalculate |el|'s [=inset-modified containing block=],
+				with any non-''top/auto'' [=inset property|inset=] values
+				shifted by the [=snapshotted scroll offset=].
+				Set |cb rect| to this result.
+
+			3. If |cb rect| was negative-size in either axis
+				and corrected into zero-size,
+				[=iteration/continue=].
+
 		4. If |el rect| is not fully contained within |cb rect|,
 			[=iteration/continue=].