The ekr-undo branch is a continuation of
#1413.
PR 1759 shows the new work.
The new code simplifies and generalizes undo-related code. See the first comment of the PR for full details.
As with previous changes to the undo code, there will surely be microscopic differences between the old and new code. I expect the new code will be better in almost all cases. Indeed, the new code enforces new policies and new checks.
Please test this branch asap. We aren't going back to the old code. I'll merge the ekr-undo branch into devel in a few days.
Edward