I've committed a fix that reduces the flickering in the menubutton widget in general, as well as improving performance during resizing of windows; the widget now looks fine when mapped. The widget still inexplicably renders during window resize when it is hidden, and I do not know how to get that resolved. You may still be better off working around the issue by using ttk menu buttons or destroying/redrawing the widget, as that seems to work fine.

Please confirm that the update performs as I've indicated and I'll close this bug.