what is the status of a scrolled-out widget ?

[Paul Barton-Davis <pbd Op Net>]

if you pack a widget into a box, and put the box into a scrolled
window: when you scroll so that widget vanishes, what has happened to
it ? it appears not to be unmapped, and it also still claims to be
visible. what's the trick ? 

i want to label the end points (and only the endpoints) of the set of
widgets currently viewable in a scrolled window, but i can't figure
out how to differentiate those widgets that are truly visible and
those that just think they are.

--p