I tried to reproduce the problem locally but everything is worked as expected. Please find attached a test case along with a short movie showing what is happening on my side.
Note that the RadWindow control is basically an IFRAME - it does not affect the controls in the content page in any way. This being said, I suggest to replace the RadWindow with a standard IFRAME and see if the problem exists. If it can be reproduced only when the content page is opened in a RadWindow - please open a support ticket and send us a sample project that reproduces the problem so we could check it.
All the best,
the Telerik team