bug#51521: Emacs-28 and Master. Unwanted display artifacts in multiple column tty displays.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs and Eli.

In the master and emacs 28 branches, on a Linux console or a maximised X
Window, do this:

(i) emacs -Q -nw
(ii) C-x 4 C-f <some file with plenty of blank lines> RET
(iii) M-> ; to end of buffer
(iv) Hold down <up> (or ? C-p).

As point passes the top of the window, this will cause the window to
scroll.  Notice that the divider between the left and right windows
loses glyphs - they gradually become replaced by spaces.  This is a bug.

Further observations:
(i) If point is moved to the left hand window with C-x o, the divider is
  displayed correctly again.
(ii) After (i), and moving point back to the right hand window, the bug
  cannot now be reproduced any more.
(iii) After (ii), move point to LH window and C-x 1, deleting the RH
  window.  C-x 4 b <file> RET to recreate the RH window.  The bug is
  apparent again.
(iv) The places the divider glyphs vanish correspond to blank lines in
  the buffer in the RH window.
(v) With three windows side by side, and the <file with blank lines> in
  the RH window, both dividers lose glyphs, largely but not entirely in
  synch with eachother.
(vi) The bug doesn't occur in a GUI session in X.  It also doesn't
  appear to happen in emacs -Q -nw when the window isn't maximised.

A git bisection shows that the following commit introduced the bug:

commit 10753bc6888c997c31408a3ec59df42a4bef0005
Author: Eli Zaretskii <eliz@gnu.org>
Date:   Tue Jul 6 20:11:51 2021 +0300

    Fix right-margin display on TTY frames

    * src/dispnew.c (prepare_desired_row, adjust_glyph_matrix): Adjust
    the glyph pointer of the right-margin area for all windows but the
    rightmost ones on TTY frames, to account for the border glyph.
    (Bug#48257)

 src/dispnew.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

-- 
Alan Mackenzie (Nuremberg, Germany).