Initially RibbonView gets rendered design-time. Trying to edit RibbonTab-s or add new one causes the whole control to dissapear in the designer. Clean and rebuild of the solution doesn't help -- cleaning the Shadow cache of Visual Studio Designer is the onliest thing that helps.

The issue reproduces with Class Library containing the Ribbon setup and IsWindowsThemeEnabled property of RibbonWIndow turned off (default value).