Do you know Kakoune?
I've used a month and like it. Fast, lean, work with other unix tools well, extend by sh,
Appreciate the tip, though!
Yes, but there's a GTK proof of concept: https://gitlab.com/Screwtapello/kakoune-gtk
> Anything comparable to emacs "shell" mode?
No, it's design goal is to be just an editor, things like shell mode are left to something like tmux or a tiling window manager.
> Is it easy to extend?
Yes, but not in the same way as emacs or vim, it's very much built around the unix as an IDE philosophy so it makes interacting with external tools easy through shell scripts.