With the fullscreen refactors in place, I got started on the thing that I wanted to do all along: maximize.
Niri didn't support normal Wayland maximize because it's very similar (yet slightly different) to our full-width columns—and can't be bound to our full-width state either. However, after plenty of requests, and thinking about it, I reconsidered. Henceforth, the maximize buttons and double-clicking on the titlebars will do the expected thing.
Details in PR: https://github.com/YaLTeR/niri/pull/2376