dialog overlay blocks root html page element's scrollbar in Gecko browsers

When one uses dialog with it's overlay feature (modal dialog), it blocks ability to scroll the whole page with it's scrollbar. I mentioned this behavior on Google Chrome (ver. 19.0.1084.56 m) and Safari 4 (530.17), but not in FF, IE or OPERA. It looks like all browsers, but those using WebKit (and, maybe, KHTML) engine, dont allow scripts to prevent default event handler for the root HTML element scrollbar. It's not that big problem, but still is inconsistency.

These lines in dialog's script file cause behavior in question:

setTimeout(function() {
	// handle $(el).dialog().dialog('close') (see #4065)
	if ($.ui.sg_dialog.overlay.instances.length) {
		$(document).bind($.ui.sg_dialog.overlay.events, function(event) {
			// stop events if the z-index of the target is < the z-index of the overlay
			// we cannot return true when we don't want to cancel the event (#3523)
			if ($(event.target).zIndex() < $.ui.sg_dialog.overlay.maxZ) {
				return false;
			}
		});
	}
}, 1);