> Without thread support, event generation from multiple threads fails immediately.

This ticket is for threaded Tcl only, so this is off topic.

In nonthreaded Tcl, this script crashes rather than freezes, for an entire ly different reason that I already explained in https://bugs.python.org/issue33257 .

This ticket is solved if you ask me.
The only remaining matter is that there's no documentation:

* on Tkinter threading model: https://docs.python.org/3/library/tk.html claims full thread safety which is untrue.
* on best practices with Tkinter: as you could see, all the more or less obvious solutions are flawed. (That includes my solution: the program doesn't terminate gracefully if you close the window by hand.)

I'm going to cover at least the first item as part of executing Guido's suggestion to "add a warning to the docs".