To reproduce, open the ribbon sample and minimize the ribbon bar.
Now move the application to the right or left side until parts of the window are outside of the display area and open a ribbon group.
The group is outside of the application window to make sure that all parts are visible. But the expected bahaviour would be that not everything is visible. Other tools like Office don't move the group outside of the application window. The ribbon is just clipped like the rest of the application.
Best regards, Tobias Lingemann.