bug#9159: 24.0.50; `undo' is not as good as it should be wrt property changes

[Drew Adams]

> >> > C-M-_ to undo the last change.
> >> > With point in the same place, do `C-u C-x ='.
> >> > That shows that there is property `foo' there, with value nil.
> >>
> >> Do you have an example situation where the difference matters?
> >
> > What does "matters" mean?
> 
> By "matters" I mean that the difference is reflected in a different
> behavior for "the end user".  I'm sure you know what I mean.

I _described_ a visible difference for the end user.  That is pretty much the
only things I care about in general: user-visible behavior.

What is not user-visible in the behavior I reported?  The whole point of the
report was about the behavior being confusing to an end user.

> > Evidently it does not matter much to you.
> 
> Not until I see that it has some more serious consequence.  What you
> show is a mostly cosmetic difference.

Call it such a name if that makes you feel better.
It is a user-visible difference that promotes confusion.

No, it is not the most important problem Emacs has.
And it is unlikely to cause mass destruction.

> >> Usually we handle the absence of a text property as 
> >> synonym to a nil value of that property.
> >
> > Yes, I know that.  And this is a good example where it makes
> > a difference.

A user-visible difference, I might have added.

> If you accept the fact that "nothing == nil" for text-properties, then
> the difference you show is indeed not a difference, it's just
> a cosmetically different way to represent the same state.

Whatever.  If you don't think this difference in what the user sees and what
s?he is likely to expect/understand promotes confusion, or if you see it but
don't care, nothing I say will change your mind.

I already said in the OP that "while correct" (N.B.) "it would be better for the
property `foo' to simply be removed."

We agree that the value shown is correct.  I say that although correct it is
confusing.  You say that it's just cosmetic - bad looks.  Your poor cosmetics
can confuse users.  And that was the point, from the beginning.