bug#51580: hl-line-mode doesn't override global-hl-line-mode in current buffer

[Gregory Heytings]

> > > It seems like there is currently no way to use `global-hl-line-mode' 
> > > while disabling it for individual buffers.
> >
> > There is one: (setq-local global-hl-line-mode nil).
>
> Here, that leaves an overlay in the buffer.  Does it not do that for 
> you?
>
> [You didn't quote it, but I wrote "(setq-default global-hl-line-mode 
> nil)" in the bug report.  That was a typo.  I meant to write 
> "(setq-local global-hl-line-mode nil)".]

Oh, now I see what you mean.  Indeed I had seen your setq-default, but did 
not guess it meant setq-local.  Indeed, M-: (setq-local 
global-hl-line-mode nil) leaves an overlay in the buffer.  I use this in 
buffer setup hooks, so the overlay is not created.  If you want to remove 
that overlay, you also need to call global-hl-line-unhighlight:

(defun buffer-deactivate-global-hl-line-mode ()
  (interactive)
  (global-hl-line-unhighlight)
  (setq-local global-hl-line-mode nil))