Consider usingTcl_QueueEvent orTcl_EvalEx to queue up a call to a tkinter-registered python function to avoid the polling loop. Failing that, I think this all occurs in the main thread, so you can use adeque instead of aSimpleQueue.

It is not possible to call any Tk functions due to the Tkinter-internal Tcl thread lock. I'm fairly certain I triedTcl_QueueEvent as well, but can do so again.

I was under the impression that these windowprocs are called withinTcl_DoOneEvent, usually here:

https://github.com/python/cpython/blob/1e9f0933095403b215c2c4a0be7915d034ff7026/Modules/_tkinter.c#L2938

so I was expecting it to hold thetcl_lock already. If that's not the case... sorry for the distraction!

Actually no, I think I triedTk_QueueEvent which does not work because of said locks. Looking at the API forTcl_QueueEvent, I'm not sure how it could be used. It takes a pointer a function to call later, which would still be my code and unable to (un)lock anything in the Tkinter library.

Note that Tk/Tcl is threaded on Windows, so it doesn't go through that route, but this one:
https://github.com/python/cpython/blob/1e9f0933095403b215c2c4a0be7915d034ff7026/Modules/_tkinter.c#L2931

As an alternative, I tried callingTcl_SetVar on a variable named per-window, and this might work without crashing. I need to do a bit more testing to be sure, since concurrency issues can be heisenbugs.

OK, I used a Tk variable now; this is nice as it cleans up the queue polling.

I think what happened originally is that I tried the variable approach from Python, and that failed due to the competing locks which I only found out about some time later.

I like it, way better than the original and less magical than what I was imagining.