bug#26920:

[Eli Zaretskii]

> From: Yuri Khan <yuri.v.khan@gmail.com>
> Date: Sun, 14 May 2017 15:45:04 +0700
> 
> $ emacs -Q
> 
> Observed behavior: *scratch* buffer with two comment lines.
> 
> (set-face-attribute 'font-lock-comment-face nil
>   :box (:line-width -10 :color "yellow")) ^J
> nil
> 
> Observed behavior: The text in the two comment lines moves 10px to the
> right, indicating that vertical lines of the box are drawn outside
> character area.
> 
> Documented behavior — (info "(elisp) Face Attributes"), emphasis mine:
> 
>     A negative width -N means to draw a line of width N
>     that occupies the space of the underlying text,
>     thus avoiding any increase in the character height **or width**.
> 
> Documented behavior — (describe-function 'set-face-attribute):
> 
>     If WIDTH is negative, the absolute value is the width
>     of the lines, and draw **top/bottom lines**
>     inside the characters area, not around it.
> 
> So, function documentation is accurate while Info text is technically
> accurate but misleading (the character widths do not change, but the
> total width of the run of text does).

Thanks, fixed.