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.
The check mark of the RadioButton is misaligned when DPI settings are set to a higher than the default value, e.g. 125%.
The problem can easily be reproduced using the Telerik UI for WPF Demo application: