When there are enough items in a RibbonBackstageItem for the ScrollBar to appear, it is clipped by the RadRibbonWindow's header.

Workaround:
1. Extract the templates for RadRibbonBackstage and ScrollViewer
2. Refer the new template for the ScrollViewer in the template for the RadRibbonBackstage.
3. Change the margin for the PART_VerticalScrollBar from 0, -1, -1, -1 to 0, 27, -1, -1.
4. Set the new ControlTemplate to the RadRibbonBackstage in the application.