> With a canvas-based engine, the editor is no longer relying on the contenteditable spec right?

Correct. In fact, contenteditable went out the window a decade ago when the "#2" engine (low-level DOM manipulation) was launched.

My experience with contenteditable is ~12 years stale at this point, so the only thing I'll try to say is that I expect it would work well up to a certain level of ambition, and no further. As I say above regarding frameworks: they're great so long as your requirements fit within the expectations of the framework, but you quickly hit a wall if you need to stray outside of that. For Docs, the desire for a paginated view/edit mode was an example; there was simply no sane way of squeezing pagination into a contenteditable-based engine.