bug#68314: 30.0.50; incorrect redisplay of contents of current line

bug#68314: 30.0.50; incorrect redisplay of contents of current line

[Richard Stallman]

Here's the input I typed (at the end of a line containing
"US streaming dis-services have deleted many programs.  ").
Then I saw that the `e' at the end of `article' was missiing.

 SPC     ;; self-insert-command
 T       ;; self-insert-command
 h       ;; self-insert-command
 i       ;; self-insert-command
 s       ;; self-insert-command
 SPC     ;; self-insert-command
 a       ;; self-insert-command
 r       ;; self-insert-command
 t       ;; self-insert-command
 i       ;; self-insert-command
 c       ;; self-insert-command
 l       ;; self-insert-command
 e       ;; self-insert-command
 SPC     ;; self-insert-command
 g       ;; self-insert-command
 i       ;; self-insert-command
 v       ;; self-insert-command
 e       ;; self-insert-command
 s       ;; self-insert-command
 RET     ;; newline
 m       ;; self-insert-command
 a       ;; self-insert-command
 n       ;; self-insert-command
 y       ;; self-insert-command
 SPC     ;; self-insert-command
 d       ;; self-insert-command
 e       ;; self-insert-command
 t       ;; self-insert-command
 a       ;; self-insert-command
 i       ;; self-insert-command
 l       ;; self-insert-command
 s       ;; self-insert-command
 .       ;; self-insert-command
 ESC DEL ;; backward-kill-word
 e       ;; self-insert-command
 x       ;; self-insert-command
 a       ;; self-insert-command
 m       ;; self-insert-command
 p       ;; self-insert-command
 l       ;; self-insert-command
 e       ;; self-insert-command
 s       ;; self-insert-command
 .       ;; self-insert-command

So I tried to fix that

 C-p     ;; previous-line
 C-e     ;; move-end-of-line
 ESC b   ;; backward-word
 C-b     ;; backward-char
 e       ;; self-insert-command
 C-l     ;; recenter-top-bottom
 DEL     ;; delete-backward-char

and found that the `e' was missing only on the screen -- the buffer
was correct.  Therefore I typed DEL.

This bug in redisplay has been happening every few minutes this
morning.  I did not notice it happen yesterday, which surprises me
now.

This example is the simplest of those I have looked at recent-keys
for.  It has NOTHING unusual!

I have a suspicion.

I get tiny pauses even in typing ordinary input, as if something
time-consuming were running frequently between commands.

Those tiny pauses could affect how redisplay updates for these
inserted characters, in a random, unpredictable way.

I suspect this bug usually happens in the part of redisplay
that updates the current line onl.  Once I typed DEL and
a character two columns leftward was erased.

On the other hand, sometimes redisplkay fails to rewrite some lines,
below point, and leaves then blank or with old contents.

The problem seems to happen mainly in mail mode.



In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 2.24.32, cairo version 1.16.0) of 2024-01-02 built on freetop
Repository revision: 1c369263db826a02e01d4209c7b6b8617971c37c
Repository branch: master
System Description: Trisquel GNU/Linux Nabia (10.0)

Configured using:
 'configure 'CFLAGS=-O0 -g' --with-gnutls=ifavailable'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GPM GSETTINGS HARFBUZZ JPEG JSON LIBOTF
LIBSELINUX LIBXML2 M17N_FLT MODULES NOTIFY INOTIFY PDUMPER PNG RSVG
SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XINPUT2
XPM GTK2 ZLIB

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

Major mode: RMAIL

Minor modes in effect:
  gpm-mouse-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
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  auto-composition-mode: linux
  auto-encryption-mode: t
  auto-compression-mode: t
  abbrev-mode: t

Load-path shadows:
None found.

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kinsoku url-file svg xml dom dabbrev mailalias rmailout rmailkwd qp
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Memory information:
((conses 16 711365 165293) (symbols 48 21608 35)
 (strings 32 121559 15771) (string-bytes 1 2901740) (vectors 16 97591)
 (vector-slots 8 2218489 87800) (floats 8 283 617)
 (intervals 56 60611 1987) (buffers 976 93))
[[[ To any NSA and FBI agents reading my email: please consider    ]]]
[[[ whether defending the US Constitution against all enemies,     ]]]
[[[ foreign or domestic, requires you to follow Snowden's example. ]]]


-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)

bug#68314: 30.0.50; incorrect redisplay of contents of current line

[Eli Zaretskii]

> From: Richard Stallman <rms@gnu.org>
> Date: Sun, 07 Jan 2024 22:44:59 -0500
> 
> 
> and found that the `e' was missing only on the screen -- the buffer
> was correct.  Therefore I typed DEL.
> 
> This bug in redisplay has been happening every few minutes this
> morning.  I did not notice it happen yesterday, which surprises me
> now.
> 
> This example is the simplest of those I have looked at recent-keys
> for.  It has NOTHING unusual!

It is hard for me to believe that there's such a fundamental problem
in redisplay.  My suggestion is to use open-termscript to record what
the display code sends to the screen, and when the problem happens
again, look for the stuff you miss on the screen in the termscript
file.  Then we will know whether the problem is that redisplay didn't
send the correct commands to the screen or that the commands were sent
correctly, but were not executed correctly by the terminal driver.

> I have a suspicion.
> 
> I get tiny pauses even in typing ordinary input, as if something
> time-consuming were running frequently between commands.
> 
> Those tiny pauses could affect how redisplay updates for these
> inserted characters, in a random, unpredictable way.

How could some processing affect redisplay?  If you have an idea how
that could happen, please tell, because I cannot imagine how it could
happen.  It is much easier for me to suspect that your system's video
driver or some other system software related to the terminal
"swallows" commands sent to the terminal.

What happens if you type "M-x redraw-display RET" -- do the "missing"
characters come back?  What happens if you suspend Emacs and then
resume it?

> I suspect this bug usually happens in the part of redisplay
> that updates the current line onl.  Once I typed DEL and
> a character two columns leftward was erased.
> 
> On the other hand, sometimes redisplkay fails to rewrite some lines,
> below point, and leaves then blank or with old contents.
> 
> The problem seems to happen mainly in mail mode.

Did you recently change anything in your system's software, like
upgraded some drivers or maybe even moved to a new system?  Did you
update your terminfo DB or something related to it?