Yes, this is exactly the same problem as `issue42225`. I suspected it was a Tkinter problem, and also thought that Python might be under the hood catching C method call errors. 

But since you know about this problem, and in more detail than I have written here, this issue can be closed.

P.S. By the way, while I was doing my little investigation with fonts, here's another little bug I found :) 
When I start to write any word in IDLE and press Tab, the autocomplete box is shown (or auto-complete if the only mapping). But if I was doing something in "Configure IDLE" and pressed "Ok" to close the configure window, then pressing Tab just causes tab to appear at the cursor instead of auto-complete.

You probably know about it, but You know, I'm something of a developer myself, so it's better to report a bug than not report it.