The problem exists because we have set width of 100% to window's content area:
height: 100% !important; /* very important property, especially for opera */
background-color: #fff !important;
Add the rule set from below to your theme to fix the problem. We have also fixed the issue in Sitefinity developer branch and the fix will be available in one of the next internal builds releases.
body .RadWindow .rwWindowContent
body .RadWindow.rwMaximizedWindow .rwWindowContent, body .RadWindow.sfMaximizedWindow .rwWindowContent
Let us know if you need further help.
the Telerik team
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