As of jQuery 1.3.1 and UI 1.6rc6, triggered mousedown event on draggables throws an error

As of jQuery 1.3.1 and UI 1.6rc6, triggered mousedown event on draggables throws an error


Last night I upgraded from jQuery 1.2.6 and UI 1.5.3 to jQuery 1.3.1
and UI 1.6rc6 without too many problems at all (which is fantastic!).
The only problem I've found is when firing a mousedown event on an
element that has draggable (or potentially, any mousedown) behaviour
attached. This now throws an error on the following line of
$.ui.mouse._mouseDown, due to 'originalEvent' not being present:
if (event.originalEvent.mouseHandled) { return; }
Here's a cut-down test harness which illustrates the problem nicely:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://
www.w3.org/TR/html4/loose.dtd">
<html>
<head>
    <title>jQuery Event Test</title>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <script type="text/javascript" src="assets/js/jquery-1.3.1.js"></
script>
    <script type="text/javascript" src="assets/js/jquery-ui-
personalized-1.6rc6.js"></script><!-- core, mouse, draggable,
resizable -->
    <script type="text/javascript">
        jQuery(function() {
            jQuery('#mouseFireManual').bind('click', function(eventData) {
                jQuery('#draggableDiv').triggerHandler('mousedown', eventData);
            });
            jQuery('#draggableDiv').bind('mousedown', listenForMouseDown);
            jQuery('#draggableDiv').draggable({});
        });
        function listenForMouseDown() {
            document.title = 'Mouse down handler was fired ' + new Date
().getTime();
        }
    </script>
</head>
<body>
    <div id="mouseFireManual">Fire mouse down event</div>
    <br />
    <div id="draggableDiv">This DIV is draggable</div>
</body>
</html>
If you click the 'Fire mouse down event', you'll get an error, but you
drag 'draggableDiv' without any problem.
Switching in jQuery 1.2.6 and UI 1.5.3 with no other code changes, and
the error goes away.
Is this something I should report as a bug, or is this expected
behaviour with the new event model?
Thanks!
Dan