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

> 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.

1. emacs -Q --load minimal.el
2. M-x tab-bar-mode
3. Scroll (pixel scrolling).
   Let x = fractional part of number of lines scrolled.
4. C-t to toggle off header line.
5. The lower x fraction of the header-line takes over the
   lower x fraction of the tab-bar until the tab-bar is redrawn.


Corwin