I cannot call the Canvas method _update_idletasks() from within
_Screen.setup() becaus this would contradict to the architecture of the
module which isolates all direct references to Tkinter to
TurtleScreenBase. (The idea behind this is to make the module easily
portable, by porting only this class).

So if one followed your proposition one had to change TurtleScreenBase
which would result in a different (additional) patch.

For now I'd like to stick with the one proposed here - I worked a lot
with it, it works and I didn't experience unwanted side effects.

My question: I suppose to change the call self.cv.update() to
self.cv.update_idletasks() in TurtleScreenBase._update wouldn't work
properly so one had to add a new method to TurtleScreenBase - something
like TurtleScreenBase._update_idletasks. Originally I had the intention
to keep this interface as small as possible and use only things that are
really needed. Did you experience any problems or undesired behaviour
because of using unly cv.update? If so, please report it. Perhaps you
could also provide an example.

Regards,
Gregor