Undo does not work in Notebook Cells (confusion around cell level undo/redo stack)

Does this issue occur when all extensions are disabled?: Yes/No

• VS Code Version: Insiders
• OS Version: Mac

Steps to Reproduce:

1. Open an existing ipynb notebook
2. Copy the last cell down (duplicate it)
3. Hit the z key (it should get deleted)
4. This works
5. Next duplicate the cell again & click else where in the notebook (empty area below the last cell)
6. Now set focus back to the last cell and hit z
7. Nothing happens
8. From now on, z will never work, undo command from the command palette too doesn’t work

I’ve tried enabling/disabling the setting Cell level undo/redo stack, no difference. I think there’s a confusion (discussed this in nb sync meeting) around what the Cell Level undo/redo stack does as well (or perhaps the confusion is due to the fact that this doesn’t seem to work).

TLDR: I wasn’t able to get undo/redo working reliably enough (in fact majority of the time it didn’t work as expected).

[nickeubank]

I just want to flag this as something that I think is pretty important to address – its really easy to lose a lot of work if an errant delete command deletes the wrong cell and there’s no undo.

[wwdok]

I have also found that i can only undo those operations in one cell, not the whole notebook, moreover, if i delete one cell, i can not undo it.