Right click on tab header closes tab unexpectedly

[oatkins]

If ContextMenu is styled to have no border, right clicking on an AvalonDock tab header can result in the mouse click being delivered to the first item in the context menu. Since this happens to be Close, the tab sometimes closes unexpectedly.

We have a fix for this in a local build of AvalonDock, but this is to track contributing it back to your version if that would be helpful.