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

I have a question -- I apologize for my ignorance. The current normative
definition in the current spec appears to be this:

"On getting, the visibilityState attribute MUST return the DOMString
visible if the Document contained by the top level browsing context is at
least partially visible at on at least one screen."

The counterpart in Arvind's draft is this:

"On getting, the visibilityState attribute MUST return the DOMString
visible if the Document is at least partially visible at on at least one
screen."

I'm not sure why the current spec suffices but the draft does not. Why is
that? There are additional CSS concerns, to be sure. But it may be worth
noting that, for example, in minimized windows the behavior of
visibilityState differs across browser vendors and OSes (with the caveat
that the attribute is still vendor-prefixed). The current spec seems to be
useful in spite of the incompleteness -- I know it has been useful to me.

-Walter