Which one?

Perhaps Amethyst[1] or yabai[2]. You can also write your own with Hammerspoon[3].

[1] https://github.com/ianyh/Amethyst [2] https://github.com/koekeishiya/yabai [3] https://github.com/Hammerspoon/hammerspoon

Yabai is the one I was using.

Not who you're responding to; there's Amethyst but I use Rectangle (the fork of now defunct Spectacles).