> fish --version
fish, version 4.0.6-1 (Built by MSYS2 project)
> uname
MINGW64_NT-10.0-26100
> command -s (__fish_anypython)
/ucrt64/bin/python3
> /ucrt64/bin/python3 --version
Python 3.12.11

Note: also happened with 4.0.2. I haven't tried older versions of fish

When running fish_config, as soon as the browser connects, the message "Shutting down" shows up and the browser is indefinitely waiting for the page to load.
Despite the message, fish_config does NOT shutdown until I press ENTER. At that point I get the shell prompt and the browser shows a "Unable to connect" error message.

I looked quickly at webconfig.py and the shutdown happens because sig_fileno is in ready_read instead of http_fileno.

ready_read: [576], [], []
http_fileno: 444
sig_fileno: 576

As for fish_config not immediately returning to the prompt, it's because the app is blocked on the last thread.join()