Ronning out of Undo

[SMcNeill]

Then can't the Undo function compare the before and after versions, and report if no changes were made?

If the program doesn't change, then the undo isn't going to change.   If no changes are made, there's not going to be a "before and after" to report on.

PRINT "foo"

Type that in, and you have 2 basic undo points:
PRINT
PRINT "foo"

Unless that PRINT "foo" changes, that undo file isn't going to change either, as it's only going to log when you make a change.  Everytime you undo, a change is going to happen.