By the time you’ve fixed as much as you can, you will certainly have more code, more complex code and more fragile code than you would have had from massaging contenteditable into your desired shape, and your result won’t even be better than it in any indisputable way.

“Pure canvas” means when the entire app runs inside a canvas to the maximum extent possible, such as with https://www.egui.rs/. Google Docs isn’t pure canvas because it uses regular DOM for the chrome, leaves text rendering to the browser (fillText or whatever) and uses real scrolling area (multiple canvases, tiling). Yours is similarly not pure canvas, though it’s a little closer in the obviously-bad way of how you implement scrolling yourself.

“Google did it” is not an endorsement for others doing it. They have often quite bizarre rationales that truly don’t apply to smaller organisations. As I have written, I’ve thought it through carefully multiple times in multiple ways, and I honestly believe they were lying about their motives through and through, because they just don’t make any sense. Google Docs was unambiguously better before they migrated to canvas, and most of their claims about what it got them were demonstrably false.