I ended up implementing my own background process management for fish here, it uses bash to fork a subshell with fish inside of it, which seems to solve all the problems I encountered.

https://github.com/pirate/fish-functions/blob/master/backgro...

https://github.com/pirate/fish-functions/blob/master/progres...

https://github.com/pirate/fish-functions/blob/master/signal....

https://github.com/pirate/fish-functions/blob/master/fork

Usage:

    background progress_await compiling_task_finished

    g++ ... some slow command

    signal compiling_task_finished

This allows the progress bar to truly run at the same time as the g++ command and receive a signal from the fg process when it's time to complete, something a simple & does not let you do in fish.

I've collected more details on Fish's issues here for anyone interested:

https://github.com/pirate/fish-functions#issues-with-fish