bug#11774: org-mode causes undo boundaries to be lost

[Toby Cubitt <toby-dated-1341784362.70a4d9@dr-qubit.org>]

On Sun, Jun 24, 2012 at 11:39:08PM +0200, Bastien wrote:
> Hi Toby,
> 
> Toby Cubitt <tsc25@cantab.net> writes:
> 
> > For some unknown reason, org-mode is causing the undo boundary between
> > the (2 . 4) and (1 . 2) entries to be removed from `buffer-undo-list'.
> 
> Can you try again with
> 
>   (setq org-self-insert-cluster-for-undo nil)
> 
> and report?

Yup, that fixes the problem.

I don't fully understand the purpose of
`org-self-insert-cluster-for-undo', given that the Emacs command loop
already groups consecutive undo entries together, but presumably it
enables a more aggressive form of clustering.

If the behaviour I reported is the intended behaviour with this option
set, perhaps the clustering heuristic can be improved to avoid triggering
history discarding in undo-tree-mode?

undo-tree-mode puts a "canary" at the end of buffer-undo-list, to detect
when Emacs discards undo history behind undo-tree-mode's
back. I.e. before any undo entries have been added, buffer-undo-list
contains:

  (nil undo-tree-canary)

org-mode's undo clustering deletes the undo boundary before the
undo-tree-canary entry, causing undo-tree-mode to think that Emacs has
discarded undo history behind its back.

I could try to work around this in undo-tree-mode, but it seems to me
that org-mode shouldn't be throwing away an undo boundary that comes
before an entry which definitely shouldn't be clustered with anything
(namely the undo-tree-canary symbol, which is meaningless to org-mode).

Toby
-- 
Dr T. S. Cubitt
Mathematics and Quantum Information group
Department of Mathematics
Complutense University
Madrid, Spain

email: tsc25@cantab.net
web:   www.dr-qubit.org