We are not aware of problems with RadWindow in IE11. I have just tried the demo you linked under Windows 8 and IE11 and I am attaching here a short video from my test where everything works as expected.
Could you examine it and let me know if I am missing something? It is possible that your machine has connectivity issues with our CDN provider so some scripts are simply not loaded. Could you check if this is the case? If there are errors in the console as well, could you share them with us? If you can show us how to reproduce a problem we will investigate it as soon as we can.
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