Catching autopostback in client script before submitting

Hello all,

I'm trying to set up a javascript manager for preventing form
submitting in certain cases. I have set up an interceptor that catches
postbacks and cancels them if needed.

This mechanism works just fine, except that with autopostbacks, the
intercepted postback never fires. For some reason, the postback is
just run.

Observing onbeforeunload event is unfortunately out of the question
due to UI requirements, unless someone is able to figure out how to
prevent the automatic prompt caused by cancelling the event.

TIA,

sdr