[css-anchor-position] Ensure that inset-area gets the same scroll-shi…

…fting that manual inset usage does.

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