bug#40521: 28.0.50; :box attribute in header-line truncates header-line-format text

[Eli Zaretskii]

> From: Andrey Orst <andreyorst@gmail.com>
> Date: Thu, 9 Apr 2020 14:50:17 +0300
> 
> I've checked some of my older screenshots and there was nothing
> like that, "Treemacs" was fully visible. After a bit of digging I've
> figured out that that's the :box attribute causing this.
> 
> Minimal recipe:
> 
>     emacs -q --eval "(progn (setq header-line-format \"1 2 3 4 5 6 7 8 9 10 
> 11 12 13 14 15 16\")
> (face-remap-add-relative 'header-line :box '(:line-width 20)))"
> 
> Emacs should appear with pretty huge header-line at the top. Try to
> resize the frame. Notice that amount of dots changes. Without the :box
> dots are being truncated by the edge of the window as those should.
> With :box it happens somewhere at the middle of the window.

Thanks, I think I fixed that now.