> How would that differ from jobstart() + autocmd JobActivity (added in #475)?

It wouldn't (really) for the VimL `system_async()` I was talking about, so what I said is moot. It could be useful for faster and/or more responsive filtering, though.

> Depends on how much conflicts it would cause...

Conflicts with? Are there many PRs changing `eval.c` in the pipeline? (save for mine and yours).