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bug#39188: closed (28.0.50; w32 cursor redisplay problem)

From: GNU bug Tracking System
Subject: bug#39188: closed (28.0.50; w32 cursor redisplay problem)
Date: Tue, 21 Jan 2020 16:33:02 +0000

Your message dated Tue, 21 Jan 2020 18:32:54 +0200
with message-id <address@hidden>
and subject line Re: bug#39188: 28.0.50; w32 cursor redisplay problem
has caused the debbugs.gnu.org bug report #39188,
regarding 28.0.50; w32 cursor redisplay problem
to be marked as done.

(If you believe you have received this mail in error, please contact

39188: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=39188
GNU Bug Tracking System
Contact address@hidden with problems
--- Begin Message --- Subject: 28.0.50; w32 cursor redisplay problem Date: Sun, 19 Jan 2020 10:17:01 +0100
With the current master (and for a couple of months), I observe this redisplay problem where moving the cursor up or down, when the buffer is scrolled, the cursor may leave some ghost trace. This is with w32-use-visible-system-caret set to t. It seems the problem does not happen if this variable is set to nil.

Picture with several traces of the cursor :

Best regards,


In GNU Emacs 28.0.50 (build 1, x86_64-w64-mingw32)
 of 2020-01-18 built on MARVIN
Repository revision: 17db2bd9c17c84c647d8093a7f03f1b0a55ddcc4
Repository branch: windows
Windowing system distributor 'Microsoft Corp.', version 10.0.19041
System Description: Microsoft Windows 10 Education (v10.0.2004.19041.21)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list... [2 times]
previous-line: Beginning of buffer [21 times]
Unable to read file "c:/Home/Org/templates/level-0.org" [2 times]
previous-line: Beginning of buffer [18 times]
Making completion list...
previous-line: Beginning of buffer [30 times]
Making completion list...
user-error: End of history; no default available

Configured features:

Important settings:
  value of $LANG: fr_FR
  locale-coding-system: cp1252

Major mode: Org

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

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Memory information:
((conses 16 167083 14304)
 (symbols 48 18300 1)
 (strings 32 60912 2917)
 (string-bytes 1 2024991)
 (vectors 16 29488)
 (vector-slots 8 331793 13298)
 (floats 8 191 294)
 (intervals 56 1214 0)
 (buffers 1000 14))

--- End Message ---
--- Begin Message --- Subject: Re: bug#39188: 28.0.50; w32 cursor redisplay problem Date: Tue, 21 Jan 2020 18:32:54 +0200
> From: Fabrice Popineau <address@hidden>
> Date: Mon, 20 Jan 2020 19:41:48 +0100
> Cc: address@hidden
> Maybe it is not the most important bug (except if Windows start to tell
> emacs that there are screen readers everywhere)

I found the problem.  As part of redisplay, we sometimes scroll
rectangular portions of the window, as pixels, up and down.  When we
do so, we first turn off the cursor, but when the system caret is used
instead of the cursor, we hide and unhide the caret in a separate UI
thread.  So there's a race between the main thread which scrolls the
pixels and the UI thread that draws the caret, and that race causes
the pixels of the caret to sometimes be copied with the rectangular
area that is scrolled.

The solution is to disable the scrolling in w32_scroll_run when the
system caret is in use.  This slows down redisplay to some extent
(because w32_scroll_run is a redisplay optimization), but I don't see
any easy way around that.

Should be fixed now on the emacs-27 branch.

--- End Message ---

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