GNU bug report logs - #44448
27.1; Strange inteference between timer, modeline/header-line and buffer position in window

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From: Amai Kinono <amaikinono <at> gmail.com>
To: 44448 <at> debbugs.gnu.org
Subject: bug#44448: 
Date: Wed, 11 Aug 2021 13:49:08 +0800

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My recent test shows this also happens when you modify overlay in the buffer
frequently. Also, a timer is not needed, it happens when it's frequent
enough.

Recipe:

1. emacs -Q

2. Eval this:

(require 'paren)
(add-hook 'post-command-hook
 (lambda ()
   (let ((show-paren-mode t))
     (show-paren-function))))

   It's a setup to run show-paren-function in post-command-hook. Directly
using
   show-paren-mode also cause the bug, but the chance is way lower, as it
   delays the refreshing using a timer. It happens to me occasionally when
I'm
   coding with show-paren-mode on.

3. M-x find-library RET paren

4. M-x split-window-horizontally

5. Move to a position in the left window where there's a bunch of closing
  parens. You should see show-paren-function is highting them for you.

6. Scroll the right window to a position where you can see the matching
opening
   parens. Now show-paren-function should highlight opening parens in the
right
   window, and closing parens in the left window.

7. Go back to the left window. Hold Ctrl and press f/b/g at the same time
   several times (so you are sending C-f, C-b and C-g frequently)

8. The cursor in the left window will jump to the position of the
   right window cursor.

I've attached a gif so you could see it clealy.

At this time, I think the bug occurs when you frequently write text to
header-line, mode-line, minibuffer (see
https://github.com/kiennq/emacs-mini-modeline/issues/45), or operate on
overlays. Using a timer or not doesn't matter.

This has been bugging me for years since company-mode (uses overlay + timer)
also causes this problem. I haven't seen similar bug reports, though. Am I
the
only unlucky company user? Could someone save me from the pain?

Emacs version: GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.24.30, cairo version 1.17.4) of 2021-08-11

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[show-paren-mode-bug.gif (image/gif, attachment)]

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