Thank you for contacting the Telerik support.
The described problem is known and it can be reproduced only when the main page is being scrolled. This matter is already fixed by our developers and the stable functionality will be available in the next official release of RadControls suite.
Let me know if you need more information.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed