Robust undo/redo remains an ongoing research project. Leo Stewen's work was presented at PLF 2023 a few days ago. It turns out to be a subtle problem to really get completely right, but in my experience you can usually get passable results by letting editors default undo behaviour reverse text input.

For applications with more document-structured data, you can now produce inverse patches using Automerge.diff to go between any two points. To implement a reasonable undo in this environment you can record whatever document heads you consider useful undo points and then patch between them.

To perhaps expand on why the problem remains unsolved slightly further, there was a robust discussion about what the expected behaviour of "undo" out to be in even simple cases at the conference.