I'm guessing that performance of this relative to xterm right now isn't... the best, mainly because the way you're grabbing the viewport seems expensive. I'm curious though if you did any benchmarks?
One thing you probably really want to expose is the new RenderState API: https://github.com/ghostty-org/ghostty/blob/main/src/termina... You're doing per row line grabbing currently which is probably pretty slow. The RenderState API is stateful and produces the state necessary to create a high-performance, delta update renderer. It's what our production GPU renderers are now built on (but the API itself is compatible with any kind of renderer). It'd be better for you.
After all that, I'm very curious even at this rudimentary level what the performance on various benchmarks look like compared to xterm.js.
Excellent work!