Sync two windows of emacs together

Question

I have a snippet of code in my dotemacs file that would sync two opened buffers side by side (Thanks to Tobias), scrolling in the master buffer will result in moving the slave buffer accordingly matching the same "line" the cursor is at.

I have been trying to modify the code and have one buffer act as master when it has the focus and the other act as a slave following lead. Basically, I want then both to sync up no matter which buffer is scrolling.

Unfortunately, Applying (Xsync-window) on both buffers failed because of how tightly synced the master and slave together.

Here is the code:

(defun Xsync-window (&optional display-start)
  "Synchronize point position other window in current frame.
Only works if there are exactly two windows in the active wrame not counting the minibuffer."
  (interactive)
  (when (= (count-windows 'noMiniBuf) 2)
    (let ((p (line-number-at-pos))
      (start (line-number-at-pos (or display-start (window-start))))
      (vscroll (window-vscroll)))
      (other-window 1)
      (goto-char (point-min))
      (setq start (line-beginning-position start))
      (forward-line (1- p))
      (set-window-start (selected-window) start)
      (set-window-vscroll (selected-window) vscroll)
      (other-window 1)
      (unless display-start
    (redisplay t))
      )))

(define-minor-mode sync-window-mode
  "Synchronized view of two buffers in two side-by-side windows."
  :group 'windows
  :lighter " ⇕"
  (if sync-window-mode
      (progn
    (add-hook 'post-command-hook 'sync-window-wrapper 'append t)
    (add-to-list 'window-scroll-functions 'sync-window-wrapper)
    (Xsync-window)
    )
    (remove-hook 'post-command-hook 'sync-window-wrapper t)
    (setq window-scroll-functions (remove 'sync-window-wrapper window-scroll-functions))
    ))

(defun sync-window-wrapper (&optional window display-start)
  "This wrapper makes sure that `sync-window' is fired from `post-command-hook'
only when the buffer of the active window is in `sync-window-mode'."
  (with-selected-window (or window (selected-window))
    (when sync-window-mode
      (Xsync-window display-start)
      )))

Answer 1

This should do the trick:

(define-minor-mode sync-window-mode
  "Synchronized view of two buffers in two side-by-side windows."
  :group 'windows
  :lighter " ⇕"
  (unless (boundp 'sync-window-mode-active)
    (setq sync-window-mode-active nil))
  (if sync-window-mode
      (progn
        (add-hook 'post-command-hook 'sync-window-wrapper 'append t)
        (add-to-list 'window-scroll-functions 'sync-window-wrapper)
        (Xsync-window))
    (remove-hook 'post-command-hook 'sync-window-wrapper t)
    (setq window-scroll-functions (remove 'sync-window-wrapper window-scroll-functions))))

(defun sync-window-wrapper (&optional window display-start)
  "This wrapper makes sure that `sync-window' is fired from `post-command-hook'
only when the buffer of the active window is in `sync-window-mode'."
  (unless sync-window-mode-active
    (setq sync-window-mode-active t)
    (with-selected-window (or window (selected-window))
      (when sync-window-mode
        (Xsync-window display-start)))
    (setq sync-window-mode-active nil)))

This basically guards via sync-window-mode-active, whether Xsync-windows is already working.

You can also toggle the mode in two windows simultaneously with:

(defun sync-window-dual ()
  "Toggle synchronized view of two buffers in two side-by-side windows simultaneously."
  (interactive)
  (if (not (= (count-windows 'noMiniBuf) 2))
      (error "restricted to two windows")
    (let ((mode (if sync-window-mode 0 1)))
      (sync-window-mode mode)
      (with-selected-window (selected-window)
        (other-window 1)
        (sync-window-mode mode)))))