[shellbackground]

My super-quick checklist for new shells consist of only one point: does it decently support background tasks?

It seems that none of recently emerged shells had that point in their design:

- elvish: no (https://github.com/elves/elvish/issues/193).

- xonsh: no (https://github.com/xonsh/xonsh/issues/1959).

- fish: no (https://github.com/fish-shell/fish-shell/issues/238).