bug#28248: 26.0.50; display-line-numbers does not affect window-width / window-text-width

[Dmitry Gutov]

Hey Eli,

On 8/27/17 5:32 PM, Eli Zaretskii wrote:
> > and there is also no variable which contains the current width of
> > the line numbers.  This all means there is no way to determine the
> > width of the text area of the window.
>
> Yes, there is.  From NEWS:
>
>    Lisp programs that need to know how much screen estate is used up for
>    line-number display in a window can use the new function
>    'line-number-display-width'.

Does it work as documented? E.g., I can enable 
display-line-numbers-mode, and they will take up 4 visual columns 
(numbers plus separators), but (line-number-display-width) returns 2.

Should I always use (+ 2 (line-number-display-width)) instead? That 
would be the actual amount of "screen estate used up for line-number 
display".

Can I rely on the extra value always being 2?