The modal overlay of the RadWindow is actually an HTML element and therefore z-index values still apply for it. This means that if controls from the page have higher z-index than the RadWindow they will be above the modal overlay as well. More information on some default values and a way to change the z-index is shown here: http://www.telerik.com/help/aspnet-ajax/controlling-absolute-positioning-with-zindex.html
. You can use the dev toolbar ot the firebug plugin for firefox to easily examine the z-index values.
the Telerik team
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