The problem shows itself on macOS High Sierra in some Tk Demos, independently of Python's IDLE. So there is something completely different, to report to the Tcl/Tk community. Strangely, I didn't notice the issues on Sierra, but I have no Sierra Mac right now to check it again. Even in Tk 8.5.18 (ActiveState or self-compiled) there are issues (notably the cascade menus demo, the button menus demo, others to) in rendering some UI elements. Sometimes bad indexes are reported (sort of [lindex {A LIST} index $N] gives $N not found even when in range)...