One simple thing is also the following:
Since Xojo Web 2.0 uses jQuery, you can simply also use jQuery UI.
Load jQueryUI into your app over the httpheader in the app object. Then call the draggable method over ExecuteJavascript with the controlId of your webdialog and you’re done.
https://jqueryui.com/draggable/
With jquery you’re also able to hide the background. Most simple thing I think.