The history object is very, very limited and rarely works as expected because the main frame and iframes often share it. Also, postbacks sometimes are considered an entry and sometimes not.
What I would suggest is that you replace the RadWindow with a simple iframe and load you content page in it. Once you are able to get the desired functionality working with it the same approach should work with the RadWindow without modifications (if you are, of course, using the NavigateUrl, if you are using the ContentTemplate the controls are part of the main page and no new page may have been loaded). You may find useful this article
, this page
and this one
as well in working with history in ASP. These are just examples, more information about this is available freely in the Internet.
the Telerik team
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