bug#81585: 31.0.91 and 32.0.50; Crash on macos hiding header-line with pixel-scroll-precision-mode

On Wed, Aug 12, 2026 at 12:58:46AM -0400, Corwin Kerr wrote:
> > Honestly, I don't know if redisplay should be sending negative values
> > to *_scroll_run, but just in case I've tried changing copyRect:to: so
> > it can handle negative values.
> The patch stops the crash for me as well. Thanks!
> 
> The crash might be a symptom of another bug because some other
> scrolling weirdness occurs.
> 
> In both Emacs 30.2 and the patched build with the same minimal.el:
> > file minimal.el
> > ;; -*- lexical-binding: t; -*-
> > (pixel-scroll-precision-mode)
> > (defun toggle-header ()
> >   (interactive)
> >   (setq header-line-format
> > 	(unless header-line-format "Pixel scroll, then press C-t to crash")))
> > (toggle-header)
> > (bind-key "C-t" #'toggle-header)
> 
> 1. emacs -Q --load minimal.el
> 2. C-x 2 and click in lower buffer.
> 3. Scroll in lower buffer.
>    Let x = fractional part of number of lines scrolled.
> 4. C-t to toggle off header-line.
> 5. The lower x fraction of header-line covers the lower x fraction of the 
>    mode-line of the upper buffer.
> (And in Emacs 32.0.50 if mode-line-invisible-mode is on, the bottom
> line of the buffer text is covered.)
> 
> > > Could you try reproducing this on Emacs 30 as well?
> > Reproduced in Emacs 30.
> Note that the same procedure in Emacs 30.2 does not crash when
> tab-bar-mode is active.

It does, it's just more difficult to reproduce. If I make the text
huge (ctrl and mouse scroll), then scroll so a line is only partially
visible and hit C-t it will (sometimes) crash in exactly the same way.

I haven't been able to find when this started, I'm beginning to
suspect it's always been a problem. As such I think I'll just push the
fix.

Is this something we want in emacs-31? Too close to release?
-- 
Alan Third