FWIW, in a low-level cross-terminal API, I made up some concepts that seemed helpful (and there are also a few comments scattered in there, such as hints about why key mapping is less consistent than one would've guessed): https://www.neilvandyke.org/racket/charterm/#%28part._.Termi...
When discussing the possibility of implementing double width or height on VTE, someone did exactly that to finally settle how things behave. IIRC the same approach was used to find out what’s the “correct” way to do sixel graphics.
FWIW, in a low-level cross-terminal API, I made up some concepts that seemed helpful (and there are also a few comments scattered in there, such as hints about why key mapping is less consistent than one would've guessed): https://www.neilvandyke.org/racket/charterm/#%28part._.Termi...