I have the same problem described in the following post:
The only difference is that i'm having it with the Ribbon view. Whenever the application menu is dropped down beyond the limit
of the ribbon, and being drawn on top of my control (in my case - a map which is being refreshed 10 times each second) the part
that is on top of the map is flickering. The solution described in the linked post helped me with the rad window, but there is no similar
property for the Ribbon view.