bug#36633: Scrolling by pixels: visible stretch glyphs overwrite header-line.

[Eli Zaretskii]

> Date: Fri, 12 Jul 2019 19:10:44 -0700
> From: Keith David Bershatsky <esq@lawlist.com>
> 
> Step 1:  Launch Emacs 26.
> 
> Step 2:  Evaluate the following code.
> 
> (progn
>   (find-library "simple")
>   (pixel-scroll-mode 1)
>   (re-search-forward "\t\t\t\t\t" nil t)
>   (set-window-start (selected-window) (match-beginning 0))
>   (setq header-line-format "hello-world")
>   (setq buffer-display-table (make-display-table))
>   (aset buffer-display-table
>         ?\t
>         (vector (make-glyph-code ?\u00BB 'font-lock-warning-face)
>                 (make-glyph-code ?\t 'highlight)))
>   (setq tab-width 8))
> 
> Step 3:  Move the mousewheel downward one time; i.e., one ratchet.
> 
> Step 4:  Observe the visible stretch glyphs overwriting the header-line.

This doesn't happen to me on MS-Windows, so I think this is
Darwin-specific.

Can someone try reproducing this on GNU/Linux and report?