bug#17235: Undo in region adjusts past positions incorrectly

[Stefan Monnier]

> I attached the patch implementing this. The new
> undo-test-region-deletion test implements the recipe of this bug
> report. It fails with the current undo-make-selective-list code,
> passes with the new.

Looks good.  Please install into trunk.

> Do you recall why the code ceases to make the selective list when an
> element cross the region?

No idea, I've never been very familiar with that code.
My guess is that the author didn't know how to make it work correctly,
or didn't want to go through the trouble of making it work correctly.

> +;; More interesting is how to adjust the "ddd" insertion due to the
> +;; next undo-delta: (6 . -2). If the reinsertion of "ad" was an undo,
> +;; it is most sensical that the total "ddd" insertion adjustment be (7
> +;; . 10) -> (6 . 8) -> (7 . 10). However, if the reinsertion was a
> +;; normal user edit, then most sensical is: (7 . 10) -> (6 . 8) -> (8
> +;; . 10). The undo history is ambiguous about which.

I was not able to understand the above comment.


        Stefan