bug#23871: 25.1.50; Undo unexpectedly leads to blank buffer

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 04 Jul 2016 17:38:52 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 23871@debbugs.gnu.org, phillip.lord@russet.org.uk
> 
> > From: Markus Triska <triska@metalevel.at>
> > Cc: Eli Zaretskii <eliz@gnu.org>,  23871@debbugs.gnu.org
> > Date: Mon, 04 Jul 2016 00:03:31 +0200
> > 
> > OK, I have constructed a shell script that simulates SWI-Prolog for this
> > specific case, please download it from:
> > 
> >     https://www.metalevel.at/ei/simulans.sh
> > 
> > Place it in the current directory and change the invocation of Emacs to:
> > 
> >    emacs -Q ceiled.pl -fn "Bitstream Vera Sans Mono 15"   --eval \
> >         "(progn (load \"$PWD/ediprolog.el\") \
> >          (setq ediprolog-program \"$PWD/simulans.sh\"))"
> > 
> > Note the setting of ediprolog-program to simulans.sh. I can reproduce
> > the issue with this script. If it does not work for you, please try a
> > different window size, font, or running Emacs in the terminal.
> 
> Thanks, I reproduced the problem, and it definitely involves some
> anomaly in redisplay.  I will look into that aspect of the problem.

This is now fixed on the master branch.  The situation where buffer
changes happen in a window whose start point is on a continuation line
was mishandled by code that never expected to find itself in this
situation.  After the fix, the recipe leaves point at the end of line
15, in a window that displays that single line at its top.

Thanks for the easy to reproduce recipe.