bug#28854: line-number-display-width erroneously returns 0 when ...

[Eli Zaretskii]

> Date: Sun, 15 Oct 2017 16:31:03 -0700
> From: Keith David Bershatsky <esq@lawlist.com>
> 
> line-number-display-width erroneously returns 0 when at the end of a very 
> long line and `truncate-lines` is non-nil.

More accurately, when the first line visible in the window is
completely hscrolled out of view.

> STEP #1:  Launch a current version of the master branch without any user 
> configuration; i.e., emacs -q.
> 
> STEP #2:  (setq truncate-lines t)
> 
> STEP #3:  (display-line-numbers-mode 1)
> 
> STEP #4:  Create a long line that is about 3 times the width of the window.  
> With point at the end of that line, evaluate:
> 
> (line-number-display-width 'pixelwise)
> 
> The result returned is 0, which is incorrect.

Thanks, fixed on the release branch.