Ask HN: What strategies are there to merge app state across devices?

That is, if someone is using an app on their phone and their computer, how is the app state resolved between the two? Or more concretely, if I were editing a text file on my computer, and I edit line 7 and add a new line (25), how could the app on my phone resolve that state if I had also been editing line 7, and added a new line?