My name is Georgi Tunev and I am from Telerik’s support team.

As you already read in the Telerik forum thread the idea of the RadWindow control is to replace the standard browser’s popups (e.g. opened with window.open()) and the control itself is based on an IFRAME. Just like the browser’s popups or the IFRAME, the RadWindow control is a container that displays a content page – different from the parent one. By design, to cover the same scenarios and to provide the same and even greater flexibility compared to window.open, RadWindow is heavily client-side oriented.

In the Telerik RadControls suite there are two other products that can be used in similar scenarios as the one you have – these controls are RadDock and RadToolTip (included in the RadControls “Prometheus” suite).

In your case, as you need to have a modal popup, I would suggest usiing RadToolTip “Prometheus” control. It allows you to set the content for the tooltip directly in the parent page just like you want.
You can check the product demo here – by the way today we will be releasing a new update which will be available on our site in several hours.

The current release of RadToolTip also has the Modal property, which if set to true will make the tooltip modal. I would suggest using a RadToolTip with the following properties set: ManualClose = true, Modal = true, ShowCallout = false.

Note that RadToolTip requires a TargetElement to position relative to. In case you need to center the tooltip rather than position it automatically relative to the TargetElement, you will need to use its clientside event OnClientShow to reposition.
Also, by default the tooltip expects to be shown relative to the mouse position, so in case you need to change its location programatically you need to also set RelativeTo=Element.

If you have any other questions, please let me know – I will be glad to help.

Best Regards,
Georgi Tunev
Telerik