Detecting whether a widget is displayed.

[Daniel Burrows <dburrows algebraicthunk net>]

  Hi list,

  I have what's probably a simple GTK+ problem, and I was wondering if
anyone could give me a tip.

  I'm trying to write a custom widget (in GTK--) that would benefit
from knowing whether it's currently visible or not.  It seems pretty
easy to detect when a widget becomes visible for the first time by
hooking into the "expose" signal, but I can't for the life of me find
any way of detecting when a widget becomes invisible.  (for instance,
when the user switches away from its tab in a notebook or scrolls past
it in a TextView)

  The most promising thing I found was the "visibility notify event",
but it never seems to get delivered to my widget.  I've also tried to
use is_viewable() and get_visible_region() on the GDK window, but even
if these work (they don't seem to so far, but I could be doing it
wrong) I need a signal or event so that I know when to test them.

  Any suggestions?

    Thanks,
  Daniel