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

[Keith David Bershatsky]

line-number-display-width erroneously returns 0 when at the end of a very long 
line and `truncate-lines` is non-nil.

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.

BACKGROUND:  I am working on developing crosshairs as part of feature requests 
#17684 and #22873, and I need the line number display width (pixelwise) to 
accurately calculate when the horizontal ruler should begin to draw -- i.e., 
it.current_x + lnum_pixel_width = start drawing ...  I am also enabling 
compatibility with the fnctions scroll-right and scroll-left, and when 
horizontal scrolling the current line.

Thanks,

Keith