Today – motivated by a challenge from a c9.io developer at a recent meetup in Seattle – I finally implemented multi-user sync-aware full document undo/redo, at least for code editors, sage worksheets, and Jupyter notebooks. If you’ve ever edited a file, worksheet, or Jupyter notebook at the same time as somebody else, and you hit control+z (or click undo) right after they type something, you would have undid their last thing. That’s because the undo/redo would use the underlying Codemirror editor’s undo/redo functionality. I wrote a new implementation of undo/redo built on top of the realtime multiuser sync functionality. Instead of undoing the last change (or changes if you undo or redo multiple times) to the document, it undoes only the changes that you made during this session.
For Jupyter notebooks in SageMathCloud this has an interesting side effect. Vanilla Jupyter itself doesn’t have any global undo – instead they have a local undo in each cell, which you could only use via the keyboard. With this change, now Jupyter notebooks in SMC have a global undo: make some changes in any cell(s), move cells around, delete cells, etc., then click undo/redo or use the keyboard to undo/redo, and the undo should undo everything you actually did across all cells.