On a system where IDLE will not start, the code has to be run directly with 'python3', however that is done there.  One will need to add the line 'r.mainloop()' at the end of the script to see the tk window.  However, this is not needed to see the printed width results.

What tcl/tk version do you distribute?  Is the person seeing the bug using that?  Add "print(tk.call('info', 'patchlevel'))" to see.

I presume that when IDLE worked for you, you used the default font.  Specific tests to run:
python3 -m test -ugui test_tk test_tcl test_ttk_guionly test_ttk_textonly test_idle