;;; show-old-window-pos.el --- Show old window position in buffer after changes.

;; Version: 2

(defgroup show-old-window-pos nil
  "Indicate the old window position for a bit when scrolling."
  :version "23.1"
  :group 'windows)

(defface show-old-window-pos-fringe 
  '((t (:foreground "cyan")))
  "Face for old window position fringe mark."
  :group 'show-old-window-pos
  :version "23.1")

(defcustom show-old-window-pos-linger-time 2
  "How long the fringe marks showing old window position should be visible for."
  :type 'integer
  :group 'show-old-window-pos
  :version "23.1")

(defvar show-old-window-pos-overlay nil)

(defun show-old-window-pos-overlay-update-pos ()
  (or show-old-window-pos-overlay
      (setq show-old-window-pos-overlay 
	    (make-overlay 
	     (window-start) 
	     (- (window-end) 1) 
	     (window-buffer))))
  (move-overlay show-old-window-pos-overlay
		(window-start)
		(- (window-end) 1)
		(window-buffer))
  (show-old-window-pos-overlay-showhide)
  (redisplay))

(defun show-old-window-pos-overlay-showhide ()
  (when show-old-window-pos-overlay
    (if (equal (window-start) (overlay-start show-old-window-pos-overlay))
	(overlay-put show-old-window-pos-overlay 'before-string nil)
      (overlay-put show-old-window-pos-overlay 'before-string 
		   (concat
		    (propertize "[" 'display 
				(list 'left-fringe 'top-left-angle 
				      'show-old-window-pos-fringe))
		    (propertize "[" 'display 
				(list 'right-fringe 'top-right-angle 
				      'show-old-window-pos-fringe)))))
    (if (equal (- (window-end) 1) (overlay-end show-old-window-pos-overlay))
	(overlay-put show-old-window-pos-overlay 'after-string nil)
      (overlay-put show-old-window-pos-overlay 'after-string 
		   (concat
		    (propertize "]" 'display 
				(list 'left-fringe 'bottom-left-angle 
				      'show-old-window-pos-fringe))
		    (propertize "]" 'display 
				(list 'right-fringe 
				      'bottom-right-angle
				      'show-old-window-pos-fringe)))))))

(define-minor-mode show-old-window-pos-mode
  "Toggle show-old-window-pos-mode"
  :global t
  :group 'show-old-window-pos
  :init-value nil
  (if show-old-window-pos-mode
      (progn
	(run-with-idle-timer 0 t 'show-old-window-pos-overlay-showhide)
	(run-with-idle-timer show-old-window-pos-linger-time
			     t 'show-old-window-pos-overlay-update-pos))
    (cancel-function-timers 'show-old-window-pos-overlay-showhide)
    (cancel-function-timers 'show-old-window-pos-overlay-update-pos)
    (and show-old-window-pos-overlay
	 (delete-overlay show-old-window-pos-overlay))))

(provide 'show-old-window-pos)