making page visibility a property of document instead of top level browsing context

Hi,
I've seen a few requests where developers would like to query for
visibility of their IFRAME (when the iframe is in third party context).

Today, in Page Visibility, we set document.visibilityState to "hidden" or
"visible", but it is really the visibility of the top level browsing
context that includes the given document. This information is made
available to third party IFRAMEs.

What do folks think of making document.visibilityState the property of the
document itself instead of the top level browsing context? That way you can
detect conditions like when the IFRAME is below the fold and therefore not
visible while the top level browsing context itself is visible.

Thanks,
Arvind