bug#28874: display-line-numbers-mode and split window vertically

[Konrad Podczeck]

Wth the pretest 26.0.90 version (for a Mac, obtained from 
https://emacsformacosx.com/builds), I observe the following:

(a) open a file with more than 10000 lines.

(b) turn on display-line-numbers-mode

(c) scroll down to the end of the buffer and the back to the beginning, so that 
you see only a part of the buffer where all line numbers have just two digits

(d) now do Menu>File>New Window Below

Then the range where the line numbers appear shrinks. Removing the split by 
middle-mouse clicking on the splitting mode-line makes that range increases 
again. For me, this behaviour is not convincing. Now do Menu>File>New Window 
Below another time and then in the lower half of the spilt scroll down, and you 
end up with two parts which have different with of the ranges showing the line 
numbers, which, for me, looks rather ugly.