[css-anchor-position] Ensure that inset-area gets the same scroll-shifting that manual inset usage does.

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -1786,6 +1786,18 @@ even tho they depend on layout and [=used values=].
 			2. Recalculate |el|'s [=inset-modified containing block=],
 				with any non-''top/auto'' [=inset property|inset=] values
 				shifted by the [=snapshotted scroll offset=].
+
+				If |el| has a non-''inset-area/none'' 'inset-area' value,
+				then for the purpose of the preceding calculation,
+				shift the value of ''anchor(start)'' and ''anchor(end)''
+				in the [=inset-area grid=]
+				by the [=snapshotted scroll offset=] as well.
+
+				Note: In other words, ''top: anchor(bottom); bottom: auto;''
+				and ''inset-area: bottom;''
+				should result in the same IMCB
+				for the purpose of these overflow calculations.
+
 				Set |cb rect| to this result.
 
 			3. If |cb rect| was negative-size in either axis