This is not a bug in either control. They are all HTML elements with absolute positioning and the one with a higher z-index value is shown on top. A Modal background in an HTML page is essentially some HTML element in the end, so this interaction depends on the overall site.
We have had an idea about a property ShowOnTopWhenModal, similar to the existing ShowOnTopWhenMaximized, and I have just created a public page where you can vote for it, leave a comment and track its progress: http://feedback.telerik.com/Project/108/Feedback/Details/56276-add-showontopwhenmodal-property-for-the-radwindow
. If it accumulates enough interest among our clients we will consider its implementation.
For the time being you can change the z-index of your RadWindows as shown here: http://www.telerik.com/help/aspnet-ajax/controlling-absolute-positioning-with-zindex.html
If you are using a RadWindowManager to create your RadWindows (and/or for RadAlert/RadConfirm/RadPrompt) you need to set the new z-index value in the manager's declaration.
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