`nlinum-hl' is an nlinum extension that tries to mitigate disappearing line
numbers in buffers that have been open a while (a known issue with nlinum).

; Installation:

M-x package-install RET nlinum-hl

  (require 'nlinum-hl)

90% of the time this should be all you need.

However, certain major-modes, commands and functions will still eat up line
numbers, typically as a result of using `with-silent-modifications` or
preventing jit-lock from detecting changes in general.

In that case, this package provides these functions:

+ `nlinum-hl-flush-region': flush a specific region in the current window.
+ `nlinum-hl-flush-all-windows': flush all open windows.
+ `nlinum-hl-flush-window': flush the current window.

Here are some examples of how to use them:

  ;; A shotgun approach that refreshes line numbers on a regular basis:
  ;; Runs occasionally, though unpredictably
  (add-hook 'post-gc-hook #'nlinum-hl-flush-all-windows)

  ;; whenever Emacs loses/gains focus
  (add-hook 'focus-in-hook  #'nlinum-hl-flush-all-windows)
  (add-hook 'focus-out-hook #'nlinum-hl-flush-all-windows)
  ;; ...or switches windows
  (advice-add #'select-window :before #'nlinum-hl-do-select-window-flush)
  (advice-add #'select-window :after  #'nlinum-hl-do-select-window-flush)

  ;; after X amount of idle time
  (run-with-idle-timer 5 t #'nlinum-hl-flush-window)
  (run-with-idle-timer 30 t #'nlinum-hl-flush-all-windows)

  ;; Bind it for flush-on-demand (this might be excessive)
  (global-set-key (kbd "<f9>") #'nlinum-hl-flush-all-windows)
  (global-set-key (kbd "<f8>") #'nlinum-hl-flush-window)
  (global-set-key (kbd "<f7>") #'nlinum-hl-flush-region) ; on selections

Also included are a variety of `nlinum-hl-do-*' advice functions, meant to be
attached to certain functions with `advice-add'.

Here are all the known issues and fixes (feel free to report/contribute
more):

  ;; With `markdown-fontify-code-blocks-natively' enabled in `markdown-mode',
  ;; line numbers tend to vanish next to code blocks.
  (advice-add #'markdown-fontify-code-block-natively
              :after #'nlinum-hl-do-markdown-fontify-region)

  ;; When using `web-mode's code-folding an entire range of line numbers will
  ;; vanish in the affected area.
  (advice-add #'web-mode-fold-or-unfold :after #'nlinum-hl-do-generic-flush)

  ;; Changing fonts can leave nlinum line numbers in their original size; this
  ;; forces them to resize.
  (add-hook 'after-setting-font-hook #'nlinum-hl-flush-all-windows)