In our latest release we decided to change the functionality of RadWindow's close button. Now, whenever a user clicks on the Close button the window is just hidden, e.g. it internally calls the Hide() method of RadWindow. If you want to close the RadWindow using the Close button, you should handle the Hidden event of RadWindow and then explicitly call the Close() method so that the RadWindow's reference is removed from the code.
By the way, it isn't necessary to add any RadWindow to the LayoutRoot element of your Silverlight application, because the RadWindow performs all necessary actions for the layout positioning internally. Therefore, you don't need the following line of code:
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