[css-contain-2] Use the element's overflow clip edge, which is the more relevant rect for 'might be seen'. Fixes #5271

Author: tabatkins

css-contain-2/Overview.bs

@@ -1251,7 +1251,7 @@ Suppressing An Element's Contents Entirely: the 'content-visibility' property {#
 		if <strong>any</strong> of the following conditions are true:
 
 		* The element is "on-screen":
-			its [=paint containment box=]'s border box
+			its [=paint containment box=]'s [=overflow clip edge=]
 			intersects with the viewport,
 			or a user-agent defined margin around the viewport.