Nice work. I checked that in situ image modifications via the toolbar that appears when an image is clicked are undoable. The undo makes the workings of the "Actions & Tools" menu clumsy. I would prefer to remove the mini-preview of a modification and show the whole image and let "Apply" be the default. The equivalent of "Cancel" is just a ^Z away. (Not to mention possible multiple undo's).

You single-handedly have convinced me to upgrade. I hope you get the big raise this year.

Multiple undo has been number 1 on my wish list for years. Naturally, now that it's been done, I tell what number 2 is: the ability to split the notebook view and look at two different parts at the same time. That's so I can look at functions I've defined at the top of the notebook when I go to use them at the bottom.

Yes, there is some of the information you can get to programmatically (and even triggered using Dynamic). But it's undocumented and should not be considered an interface that will be stable through future versions.

CurrentValue[FEPrivate`UndoInformation[_NotebookObject, _Integer:1]]

Every notebook has its own undo manager, so the NotebookObject reports undo information for that notebook only. Input fields also have their own undo manager, if you wanted to see the undo information for an InputFieldBox, you could pass in a BoxObject instead.

The integer is just an indication of how many levels of the stack you want returned. The default is 1 (just the most recent thing). If you want everything, just pass a big machine integer, like 999999.

Also, you can use RedoInformation to get information about the redo stack instead.

Example:

CurrentValue[FEPrivate`UndoInformation[EvaluationNotebook[], 999999]]

Congratulations John! It's an amazing advance to user experience.

For me, this was the biggest complaint from new users. And now has just gone.