bug#28844: 26.0.90; display-line-numbers-mode should call window-configuration-change-hook

[Paul Rankin]

Eli, I understand that your blindness to this bug is probably due to your own 
bias. I get it, you wrote the code, you enjoyed the previously mentioned reddit 
upvotes for it, I understand you don’t want to admit that your code has flaws. 
But you need to understand that your way of seeing things is subjective and not 
universal.

The bug is that when the user scrolls up or down, past a point where the width 
of the line numbers changes from e.g. 99 to the number width from 2 to 3 thus 
making the width of line number display change from 4 to 5, 
display-line-numbers-mode necessarily changes the window display configuration 
to accommodate this character.

However, display-line-numbers-mode doesn’t appear to tell the rest of Emacs 
about this change, so even if we all wanted to accommodate your display 
changes, as far as I can tell, we cannot.

It’s nice that you added the token function line-number-display-width, but 
aside from the bug with its return value being off by 2 (#29597), this function 
is not useful unless other lisp programs know *when* to call it.

The easiest solution, the one that baffles me why you haven’t just included 
already rather than continue to argue with me, is that when 
display-line-numbers-mode changes the window, you run 
window-configuration-change-hook.

If, for some Eli-reason, you don’t want to run 
window-configuration-change-hook, you can add a simple hook, which gets run 
whenever display-line-numbers-mode needs to change the window configuration. 
Then other lisp programs can call line-number-display-width, and provided its 
bug gets fixed and it returns the correct width, everyone is happy.

Is there anything about this that’s not clear to you?