Go to the RadWindow Restriction Zone demo.
When the demo has loaded, maximize one of the windows. Click on the RadWindow menu option. You should see the menu drop behind the maximized window at least it does here in both IE8 and Chrome.
Now, click on one of the options on the left side of the page, I picked cssplay.co.uk, but it doesn't actually matter which you pick. You'll see the window open up through the titlebar of the maximized window but you won't be able to access it without at least restoring with maximized window. I would have at least expected the newly opened window to appear on top.
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Stuart