bug#2749: 23.0.91; Visual Line Mode: incorrect line wrapping in corner case

[Stephen Berman]

On Fri, 10 May 2013 20:37:08 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
[...]
>> (I wish I understood why this problem occurred and how your patch
>> fixes it...)
[...]
> I can give you a vague idea of why the problem occurred.
[...]

Thanks for the explanation.

>> Unless someone knows of another problem with allowing
>> overflow-newline-into-fringe in Visual Line mode, I would like to
>> request that both my patch and yours be committed to the trunk
>
> Do you use Org mode or any other modes (e.g., magit) that are heavy
> users of display strings?  

I don't use either of those modes, but I have been developing a new
version of todo-mode.el for a very long time (which I plan to submit for
possible inclusion in Emacs, hopefully before the next pretest begins)
and it uses before-string overlays, on the order of hundreds per buffer
(though most are not in the visible window at any time), as well as
word-wrap and wrap-prefix.

>                            If not, then I'm not prepared to believe
> that the above is the only problem with this change.  I'd like to see
> much more reports from users of those modes that this change doesn't
> cause any trouble, before I'd agree to installing it.  Just look how
> much time it took until this problem popped up.  Since I knew the only
> change was in a single macro that is used in a small number of places,
> fixing it was a 5-min job.  But imagine that the same would happen 3
> years after committing this change, with gazillions of unrelated
> changes in the display code since then.

I may be misunderstanding you here, but it sounds like you think the
patch to enable overflow-newline-into-fringe in Visual Line mode is
already in the trunk; it isn't.  This bug report was initiated more than
four years ago by someone (not me) who noticed the odd display of a line
whose length is window-width when Visual Line mode is enabled.  Chong
Yidong pointed out that this was because fringe overflow was disabled in
Visual Line mode and he suspected "confusing results" if it were
enabled.  Then this past February, almost four years later, I posted
that I had enabled it in my local branch and had not observed any
problems.  I've been using it since then and only today hit the problem
that I reported and you quickly fixed.  Nobody using the vanilla trunk
can have run into the problem, because fringe overflow is still disabled
in Visual Line mode there.  But if it were now enabled (with my patch)
and your patch were also added (to prevent the problem I reported
today), then Org and magit users and others will be able to see if other
problems do arise.  Does this make sense, or have I completely
misunderstood you?

Steve Berman