This happens because the default z-index of the RadMenu is higher than the default z-index of the RadWindow. A full list is available here: http://www.telerik.com/help/aspnet-ajax/controlling-absolute-positioning-with-zindex.html
. The article also offers a way to change this. You can set a lower z-index for the menu to avoid setting it for every RadWindowManager / RadWindow.
the Telerik team
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