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

[Eli Zaretskii]

> 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.