This is really valuable! Just to add a bit of context: I'm not the only one using canvas for rendering. Google Docs did it years before me. Think of it this way: I've built this almost entirely alone and still get most of the basics working.

Let's start with your test. I think the reason it struggled with the Gutenberg text is that it had a lot of images. Either way, that's a bug and it should be fixed (it shouldn't load all the images at once, and there's probably too much work happening on the main thread). I really appreciate the effort you put in.

I could also use a WebGPU or WebGL context, which would certainly be faster. I think I had a working version last year, but it had problems with blurry rendering (font hinting). I don't remember the exact details now.

Overall, I'm really thankful that you pointed these out. I'll try to work through them over the next few days. I do disagree with the premise, though: canvas is more than capable of achieving this. Performance shouldn't be an issue. That said, I agree with your points about integration with context menus and other tooling.