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

[Stephen Berman]

On Sun, 19 May 2013 18:29:26 +0300 Eli Zaretskii <eliz@gnu.org> wrote:

>> From: Stephen Berman <stephen.berman@gmx.net>
>> Cc: monnier@iro.umontreal.ca,  2749@debbugs.gnu.org
>> Date: Sat, 18 May 2013 22:30:00 +0200
>> 
>> I hope Stefan's silence till now just means he hasn't yet had time to
>> give this issue due consideration....
>> 
>> In the mean time, I've encountered another problem, which has nothing to
>> do with display properties or overlays:
>
> So let me see if I understand how this works: you discover one by one
> the problems about this change, those same problems I was afraid of,
> and I get to debug them, is that right? ;-)

Is this new problem one you anticipated?  I thought you were worried
about problems arising from heavy use of display properties, but this
doesn't involve any.  Anyway, I really do wish I knew how to debug this
myself.  I tried stepping though Fvertical_motion but that has many
calls to other parts of the code and I couldn't follow it well -- didn't
know what could be skipped over, or what values to test, etc.  It looked
like point moved into the fringe after a call to, I think, read_char but
I couldn't tell if that was really the trigger (doesn't seem likely).
Of course, I would be grateful if you'd give me any help or advice about
how to debug it, but I suspect that wouldn't be less burdensome to you
than debugging it yourself ;-)

> Is there any point in this game where we say "enough", as too many
> problems were discovered for too small a gain?  Or does this go on
> forever?

Well, I hope you agree what started this is a real bug.  If you, Stefan
or other Emacs maintainers tell me it's not worth fixing, I can live
with that.  But as long as long as it's not fixed, other people may
encounter it and report it again....

>> -1. Apply both my patch and yours (Eli's).
>
> If you don't apply mine, does this problem still happen?

No, with my patch and without yours, this problem goes away.  So then
we're back to the previous problem.... ;-)

Steve Berman