Undo / redo functionality in the editor

[alice-i-cecile]

This is a very important UX feature to get right, and has complex implications for the backend architecture so we should start planning this early in the process.

To get thing started, here's a concrete proposal: vim-style undo branches. Rather than maintaining a single linear tree of changes, store the entire tree, allowing users to restore lost actions even if they later . Obviously you'll want to save this action history with the file, so then it's restored when restarting the session.

Getting the UX for this right seems like the central challenge. I like the approach presented here with Gundo, where you have a tagged visual tree. I think if you stuck this in a minimizable panel (perhaps with a multiclipboard?) this could be an excellent core quality of life feature.