I have verified that the same broken drawing occurs in i3 and in dwm,
even including the unpredictable results when toggling to and from
fullscreen or empty workspaces. So this isn't xmonad specific.
So R is broken with nonreparenting window managers in general. You should probably file a bug against R; there is not a lot that xmonad can do about it. :/ (The similar situation with some Java programs used to be worked around by finding that the JRE knew about some nonreparenting window managers, then making xmonad pretend to be one of the known ones.)
-- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms