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

[Markus Triska]

To reproduce this issue, please perform the following steps:

1. Install SWI-Prolog >= 7.2. I am using 7.3.23 in the following.

2. Copy ediprolog.el to the current directory via:

     $ wget https://www.metalevel.at/ediprolog/ediprolog.el

3. Copy ceiled.pl to the current directory via:

     $ wget https://www.metalevel.at/ei/ceiled.pl

4. With ediprolog.el and ceiled.pl in the current directory,
   start Emacs with:

     $ emacs -Q ceiled.pl -fn "Bitstream Vera Sans Mono 15" \
          --eval "(load \"$PWD/ediprolog.el\")"

   Please see the screenshot for how this looks for me:

      https://www.metalevel.at/ei/ceil1.png
      

5. Press:

       M-x ediprolog-dwim RET

   This consults the buffer. If everything works as intended, you see
   "Buffer consulted." in the message area.

6. Press:

      M-g M-g 15 RET

   This moves point at the beginning of "%?- time(....)".

7. Press:

      M-x ediprolog-dwim RET

   This evaluates the Prolog query at point. It only takes a few
   milliseconds to produce a very long line that spans several visual
   lines.

   Please see the screenshot:

      https://www.metalevel.at/ei/ceil2.png

8. Press:

      C-_

   This *undoes* the insertion of text that resulted from step (7).

9. As expected, the insertion is undone, but unexpectedly a completely blank
   buffer remains.

   Please see the screenshot:

      https://www.metalevel.at/ei/ceil3.png

  When I press C-p to move to the previous line, the expected part
  of the buffer is shown.

  Please see the screenshot:

      https://www.metalevel.at/ei/ceil4.png

In summary, step 8 leads to 2 problems:

(a) a blank buffer is shown
(b) point position after the undo is not where point was before the insertion.

Please note that #1095 ("Unexpected point position after undo") may be
related: Point after undo is sometimes at an unexpected position.

Thank you for looking into this!
Markus


In GNU Emacs 25.1.50.1 (x86_64-apple-darwin15.5.0, X toolkit, Xaw scroll bars)
 of 2016-05-30 built on mt-imac
Repository revision: 190942baeff3f541abf2a937e0fb4d3f9ea104be
Windowing system distributor 'The X.Org Foundation', version 11.0.11502000

Configured using:
 'configure --without-ns CFLAGS=-I/opt/local/include
 LDFLAGS=-L/opt/local/lib'

Configured features:
XPM JPEG TIFF GIF PNG GSETTINGS NOTIFY ACL GNUTLS LIBXML2 FREETYPE XFT
ZLIB TOOLKIT_SCROLL_BARS LUCID X11

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix