use react + three and this is all done for you: View tracking - CodeSandbox
otherwise, yes, scissor, but it will be much more than that. pointer events, both html and gl receiving events to begin with, running controls, views having their own environments, etc, none of this is trivial.
i would also suggest you don’t use multiple canvases. it will be slow & memory inefficient, you can’t share data and syncing will become hard. in the end the browser can terminate your tab whenever it wants bc the amount of canvases you can have is limited to a small number.