bug#29711: 25.2; mode-line distortion when using :box on mac os

[Eli Zaretskii]

> From: Adam Simpson <adam@adamsimpson.net>
> Date: Thu, 14 Dec 2017 13:20:20 -0500
> 
> 
> 1. Paste and then evaluate the following in the *scratch* buffer:
> (set-face-attribute 'mode-line nil
>                     :foreground "white"
>                     :background "red" 
>                     :box '(:line-width 5 :color "red"))
> (setq-default mode-line-format (list 
>               '(:eval mode-line-position)
>               mode-line-modes))
> 2. Open a long file that has thousands of lines. I used ivy.el which is
> around 4k lines.
> 
> 3. Observe the mode-line is "distorted" like so: 
> https://i.imgur.com/qxAsdfm.png

It isn't distorted here, but I'm not on Darwin.  Maybe it's a Darwin
specific issue.  The above PNG image obviously shows display obtained
not in "emacs -Q", so perhaps that too has something to do with the
distortion.

Thanks.