jquery1.3.2+ui1.7.2的dialog问题

问题：在IE6中使用jquery.ui.dialog来弹出一个DIV层，为了使DIV层能遮盖住select标签，加入了jquery.bgiframe.js，然后弹出一个模态对话框时，发现对话框的大小不能自由拖动了。去除模态对话框就可以拖动。
<html>
<head>
<link type="text/css" href="css/redmond/jquery-ui-1.7.2.custom.css" rel="stylesheet" />
<script type="text/javascript" src="js/jquery-1.3.2.min.js"></script>
<script type="text/javascript" src="ui/ui.core.js"></script>
<script type="text/javascript" src="ui/ui.dialog.js"></script>
<script type="text/javascript" src="ui/ui.resizable.js"></script>
<script type="text/javascript" src="ui/ui.draggable.js"></script>
<script type="text/javascript" src="external/bgiframe/jquery.bgiframe.js"></script>
<script type="text/javascript">
$.ui.dialog.defaults.bgiframe = true;
$(function() {
$("#dialog").dialog({
hide: 'slide',
show: 'slide',
modal: true});
});
</script>
</head>
<body>
<div id="dialog" title="Basic dialog">
<p>This is the default dialog which is useful for displaying information. The dialog window can be moved, resized and closed with the 'x' icon.</p>
<form><a href="#">download</a></form>
</div>
<body>
</html>