You should put a RadBusyIndicator as Content for the RadWindow you are opening and then the old content of the RadWindow should be set as Content for that RadBusyIndicator. This way when the indicator goes busy you will not be able to click the items in the window. The window can not be displayed under the RadBusyIndicator.
Eventually you can have somewhat custom solution like setting IsEnabled="False" on the root of your application and displaying a popup after the window is opened that has RadBusyIndicator. This will be tricky to implement and we do not support such behavior.
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