When you display a content page in the RadWindow (i.e. NavigateUrl is set), this is a separate page, different from the parent one. That page is displayed in an IFRAME and when you show an element with size bigger than that IFRAME, the frame uses scrollbars - this behavior is not related to the RadWindow control and is expected.
What you could do is:
- Use a standard dropdown instead of RadComboBox. The standard dropdown is a heavy-weight object that can be shown over IFRAMEs.
- Use RadWindow as a controls container. In this case the content will be in the main page and you will be able to show it in the way you want.
I hope this helps.
the Telerik team