onbeforeunload fires all the time

Since upgrading to v4 I have an issue where the onbeforeunload event fires on my parent window even when dismissing (cancel) a dialog, which is causing a problem for me. Surely I should not have to cancel the event propagation in standard plugins, in any case, how to do... Thanks

Tested in IE8. Easy to repro in replacebycode sample (ev.srcElement == null). Reason for catching event is to alert user if changes not saved.

            var ed = CKEDITOR.replace( 'editor1' );
            window.attachEvent('onbeforeunload', function (ev) { debugger; });