if you can see the edits being made in real time, keystroke by keystroke, that pretty much solves that problem.
As for offline editing, either don't support it (then you're not local-anything obviously) or you can have some lame workflow like "the document was changed by another user ..."
https://news.ycombinator.com/item?id=45341335 "We have a local-first app. Our approach? Just ignore the conflicts. The last change wins."
if you can see the edits being made in real time, keystroke by keystroke, that pretty much solves that problem.
As for offline editing, either don't support it (then you're not local-anything obviously) or you can have some lame workflow like "the document was changed by another user ..."