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 14:41:52 -0500
>
> Must be a issue on macos 10.12 then?

Actually, no: it's my mistake, I had been testing for the bug on the
emacs-26 branch instead of the 25.2 release.  I also see the bug on
25.2, so it seems like it's been fixed in version 26.  Can you please
try the pretest and verify that this is the case?