RadWindow uses an iframe to load external pages. How browsers treat this in their context menus is not something we can control.
IE11, for example, shows the source of the frame you clicked on, so just selecting View Source will show you the server response.
Firefox, however, always shows the main frame source.
Chrome offers a View Frame Source option in the context menu.
Any developer toolbar should also let you inspect the HTML in the iframe, so this is the approach you can use in all browsers.
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