After reading #28248[1] I’m not sure if this is now fixed on the release branch, but on 26.0.90 function line-number-display-width returns an off-by-2 incorrect value.

See attached screenshot or [2] also.

To reproduce:

1. emacs -Q (or emacs -Q -nw)
2. (display-line-numbers-mode)
3. (line-number-display-width)

Expected results:
4

Actual results:
2

Configuration:

GNU Emacs 26.0.90 (build 1, x86_64-apple-darwin17.2.0, NS appkit-1561.10 Version 10.13.1 (Build 17B48)) of 2017-11-05
macOS 10.13.1 (17B1003)

[1]: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=28248
[2]: https://imgur.com/a/kuZjA