Inverse line highlighting in Emacs (highlight the line you *were* on in the *other* buffer)

inverse_highlighting.lisp

;; Highlight line for _inactive_ windows ::
;; Credit to https://emacs.stackexchange.com/questions/14638/change-highlight-color-when-window-isnt-in-focus
;; Answerer: https://emacs.stackexchange.com/users/780/glucas
(require 'hl-line)
(set-face-background hl-line-face "#333")
(defface hl-line-inactive
  '((t nil))
  "Inactive variant of `hl-line'."
  :group 'hl-line)

(defun hl-line-update-face (window)
  "Update the `hl-line' face in WINDOW to indicate whether the window is selected."
  (with-current-buffer (window-buffer window)
    (when hl-line-mode
      (if (not (eq (current-buffer) (window-buffer (selected-window))))
        (face-remap-reset-base 'hl-line)
        (face-remap-set-base 'hl-line (face-all-attributes 'hl-line-inactive))
      )
    )
  )
)
(add-hook 'buffer-list-update-hook (lambda () (walk-windows #'hl-line-update-face nil t)))
(add-hook 'find-file-hook 'hl-line-mode)
(add-hook 'after-change-major-mode-hook 'hl-line-mode)