bug#44448:

[Eli Zaretskii]

> From: Amai Kinono <amaikinono@gmail.com>
> Date: Wed, 11 Aug 2021 13:49:08 +0800
> 
> 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.

Some code in paren.el probably moves point without unwind-protect, or
something like that.