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

[Charles A. Roelli]

> 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
> 
> 
> 
> In GNU Emacs 25.2.1 (x86_64-apple-darwin16.6.0, NS appkit-1504.83 Version 
> 10.12.5 (Build 16F73))
> of 2017-08-09 built on d12frosted.local

Doesn't happen here on macOS 10.6.  (For a "long file", I used M-x
find-library simple RET).  Could you please try from emacs -q?