This happens only when using XBAP Full Trust Application. Initially when floating a pane with mouse, the ToolWindow is misplaced. If the user release the dragged window and click the header again, works fine.
Based on your new radZIPLibrary it would be great if you could implement a universal, mvvm compatible backup library. It should provide support for multiple spanning medias, catalogs, discarding old backups after some time, encryption and backup of file ACL and registry data. This would be a perfect addon for your Diagram, Spredsheet and RichText libraries!