How To Prevent AutoPostBack When DropDownlist Is Selected Using JQuery

I want to show a confirm dialog when the user selects an item in a DropDownList. If the user presses "Cancel", I want to stop the postback. Here is the function I add to the onchange event:

function imitateConfirmUnload(event) {
if (window.onbeforeunload = null)
return true;

return confirm("Are you sure you want to navigate away from this page?

You have unsaved changes

Press OK to continue or Cancel to stay on the current page.");
}

And this is the relevant bit of code in my startup script to add the handler to the event:

[code]...

The problem is that the postback occurs even if the user selects "Cancel". If I move the handler on to the click event, it works. But it feels clunky to me.

Edit

Correction: it doesn't work onclick, because the dialog prevents selection, so when the user selects OK, no change has taken place, and no postback when you want it!

Edit 2

The DropDownList is inside an UpdatePanel so that may affect behavior.