We have not seen this problem in the RadWindow itself, out of the box. Such issues are observed in several cases that we cannot control
- when custom skins are used and not all necessary stylesheets are referenced on the page
- when some firewall or proxy blocks webresource requests. This can be checked in the developer toolbar of your browser. A way around it could be the StylesheetManager's CDN
- with IE - when its stylesheet limit is reached. More on that is available here
All of this information has been summarized in this help article
I hope this information will help you troubleshoot this case.
the Telerik team
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 their blog feed