[css-anchor-position] Take other scroll adjustments into account. w3c#9071

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -975,6 +975,14 @@ we define:
 
 	* Otherwise, the length is 0.
 
+	If the [=default anchor element=],
+	or ancestors between it and |query el|'s [=containing block=],
+	are subject to additional scroll adjustments,
+	such as from further anchor positioning,
+	or sticky positioning,
+	etc.,
+	add that adjustment to the [=snapshotted scroll offset=] as well.
+
 	Issue: Define the precise timing of the snapshot:
 	updated each frame,
 	before style recalc.