Douglas Engelbart in 1968 demonstrated an audio-video call between two computers, where one person were "screen-sharing" their view of a shared text file. Viewer could point at text with their mouse!!!! https://www.youtube.com/watch?v=8UQyQ7Gvi4U&t=286s
Demo Reel 3 - New try

YouTube
Hat-tip to the Zed editor blog, where they talk about Conflict-free Replicated Data Type (CRDT) implementation for their editor that allows multiple users to concurrently edit text files together. https://zed.dev/blog/crdts
How CRDTs make multiplayer text editing part of Zed's DNA - Zed Blog

From the Zed Blog: A detailed exploration of Zed's eventually-consistent text representation.