How to detect gtk widget partial visible? visibility-notify-event?

["MD Tsai" <md tsai gmail com>]

Hi list,

Is it possible for a gtk widget to detect it is partial visible? For
example, I can drag one window on another window. The covered window can be
notified that it is partial visible?

I've registered the "visibility-notify-event" but never receive callback, I
would like to figure out in my case, is it possible to receive callback? Or
the exact timing gtk will emit the signal.

Best regards,

Eric Tsai