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

From: Amai Kinono <[email-masked]>
Date: Thu, 5 Nov 2020 00:48:59 +0800

When:

- 2 windows open the same file
- There's a timer dealing with modeline or header-line format

Then pressing `C-g` sometimes scrolls the buffer in this window to the
position in the other window.

Here's a recipe for reproduce:

- `$ emacs -Q`

- Eval this:
  ```
  (run-with-timer 0 0.1 (lambda () (setq mode-line-format "hi")))
  ```

- `M-x delete-other-windows` so we have only one window.

- `C-x C-f` to open a file. I use
  <https://github.com/lsof-org/lsof/blob/master/main.c> when testing,
  but any file that's long enough should work.

- `M-x split-window-horizontally`, so we have 2 windows viewing the same
  file. For some reason, splitting vertically doesn't trigger the
  problem.

- Scroll one window down to somethere.

- Hold `C-g` or press it repeatedly and quickly, then the window will go
  to the same position in file as the other window.

You can do the same with `header-line-format`, it also has the problem.
Using `run-with-idle-timer` instead seems doesn't have the problem.

See also <https://github.com/kiennq/emacs-mini-modeline/issues/45> where
I report the same problem. I believe another package that's influenced
by this is <https://github.com/purcell/mode-line-bell>.

--- Info generated by `report-emacs-bug` ---

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.22,
cairo version 1.17.3)
 of 2020-08-29 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12009000
System Description: Manjaro Linux

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Mark set
[nil 24482 55348 93490 0.1 (closure (t) nil (setq mode-line-format "hi"))
nil nil 920000]
You can run the command ‘split-window-horizontally’ with M-x sp-h RET
Quit [22 times]
Quit
Configured using:
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 --with-modules --with-cairo --with-harfbuzz 'CFLAGS=-march=x86-64
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Important settings:
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  value of $XMODIFIERS: @im=fcitx
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Major mode: C/*l

Minor modes in effect:
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Load-path shadows:
None found.

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Memory information:
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