In order to suppress the default "Silverlight" context menu you need to hook to the MouseRightButtonDown
event of the content of the RadPane. If there isn't any, just put an empty Grid and subscribe to its event and cancel it. This is for the unpinned panes, which are in the AutoHideArea. I suppose you have already done this for the whole page or Docking control. Just keep in mind that this will suppress also if you put a custom ContextMenu.
Hope this helps.
the Telerik team
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