Unfortunately for me, even though the focus is on version 9, 8.6 allowed me to compare (I wouldn't have opened a ticket otherwise).

At first, I thought it might be due to epoll/kqueue **TIP458** (I don't know if Tk/X11 uses it) but now if you tell me that it's a limitation of Tk on macOS.

Here is a [GIF that shows the difference with my Nim program.

In any case, thank you for considering my request.I consider this to be a limitation on macOS version 9.
I would have liked to give a real example in C with the creation of an image, but I am limited in that way too.

Thank you for your detailed feedback.