Looks like a buffer list, i.e. a list of open files in the vein of tabs in mainstream editors, but with groups for those buffers. Visually reminds of Helm/Ivy's

Looks like a buffer list, i.e. a list of open files in the vein of tabs in mainstream editors, but with groups for those buffers. Visually reminds of Helm/Ivy's buffer list in Emacs. And then the whole thing got on amphetamines and the author was carried away, so now it has multilevel organization and a bunch of other features that even in Emacs are supplied by separate packages with some integration maybe. Edit: actually it's mentioned that the author has previously written about the same thing for Emacs—borrowed from someone else but rewritten from scratch.