Determining real visibility in HTML5

I'm looking for a way to determine if a region of the document is
actually visible to the end user. In particular, if the user agent
window is obscured by another window, I would want this to return false.

The rationale for doing this is when you have a web application that
performs expensive operations (either locally or by invoking server side
actions) then it is advantageous to suppress those operations until such
time as the application becomes visible (whereupon the expensive
operations can be resumed)

The 'onfocus' event isn't right as the focus doesn't have to be in a
window in order for the window to be (partially) visible.

I looked through the HTML spec, but I couldn't see anything obvious that
would help me solve this problem.

Thanks

Philip

-- 
Philip Gladstone
Distinguished Engineer
Product Development
pgladstone@cisco.com
Phone: +1 978-ZEN-TOAD (+1 978 936 8623)
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