Use window-buffer-change-function hook

     This variable specifies functions called during redisplay when
     window buffers have changed.  The value should be a list of
     functions that take one argument.

     Functions specified buffer-locally are called for any window
     showing the corresponding buffer if that window has been created or
     assigned that buffer since the last time window change functions
     were run.  In this case the window is passed as argument.

I used more naively this hook for +2 years with in mind to contribute back
sometime.  Unfortunately as time passed, I lost the memory of a reproductible
test case.  I think I implemented this after seeing #115 (Git gutter annotations
disappearing).

As I finally took the time to present you the hook, I realized that it would
mean to obsolete a bunch of functions and variables in the library such as:

 - git-gutter:update-windows-commands
 - git-gutter:pre-command-hook
 - git-gutter:update-other-window-buffers
 - git-gutter:post-command-hook
 - git-gutter:quit-window
 - git-gutter:switch-to-buffer

I also removed `git-gutter-mode` as it is generated by the minor mode and while
testing my changes, its presence caused a small bug of the mode not activating
when it should have.

git-gutter.el

@@ -3,6 +3,7 @@
 ;; Copyright (C) 2016-2020 Syohei YOSHIDA <syohex@gmail.com>
 ;; Copyright (C) 2020-2022 Neil Okamoto <neil.okamoto+melpa@gmail.com>
 ;; Copyright (C) 2020-2024 Shen, Jen-Chieh <jcs090218@gmail.com>
+;; Copyright (C) 2024-2025 Seven Beep <ebn@entreparentheses.xyz>
 
 ;; Author: Syohei YOSHIDA <syohex@gmail.com>
 ;; Maintainer: Neil Okamoto <neil.okamoto+melpa@gmail.com>
@@ -37,8 +38,6 @@
   :prefix "git-gutter:"
   :group 'vc)
 
-(defvar git-gutter-mode nil)
-
 (defcustom git-gutter:window-width nil
   "Character width of gutter window.  Emacs mistakes width of some characters.
 It is better to explicitly assign width to this variable, if you use full-width
@@ -73,14 +72,6 @@ character for signs of changes"
                (repeat :inline t (function :tag "Update command")))
   :group 'git-gutter)
 
-(defcustom git-gutter:update-windows-commands
-  '(kill-buffer ido-kill-buffer)
-  "Each command of this list is executed, gutter information is
-updated and gutter information of other windows."
-  :type '(list (function :tag "Update command")
-               (repeat :inline t (function :tag "Update command")))
-  :group 'git-gutter)
-
 (defcustom git-gutter:update-hooks
   '(after-save-hook
     after-revert-hook
@@ -508,31 +499,11 @@ Argument TEST is the case before BODY execution."
   (and (git-gutter:base-file)
        default-directory (file-directory-p default-directory)))
 
-(defun git-gutter:pre-command-hook ()
-  (unless (memq this-command git-gutter:ignore-commands)
-    (setq git-gutter:real-this-command this-command)))
-
-(defun git-gutter:update-other-window-buffers (curwin curbuf)
-  (save-selected-window
-    (cl-loop for win in (window-list)
-             unless (eq win curwin)
-             do
-             (progn
-               (select-window win)
-               (let ((win-width (window-margins win)))
-                 (unless (car win-width)
-                   (if (eq (current-buffer) curbuf)
-                       (git-gutter:set-window-margin (git-gutter:window-margin))
-                     (git-gutter:update-diffinfo git-gutter:diffinfos))))))))
-
-(defun git-gutter:post-command-hook ()
-  (cond ((memq git-gutter:real-this-command git-gutter:update-commands)
-         (git-gutter))
-        ((memq git-gutter:real-this-command git-gutter:update-windows-commands)
-         (git-gutter)
-         (unless (bound-and-true-p global-linum-mode)
-           (git-gutter:update-other-window-buffers (selected-window)
-                                                   (current-buffer))))))
+(defun git-gutter:window-buffer-change-function (window)
+  "Function to hook into `window-buffer-change-function' to update `git-gutter'."
+  (with-selected-window (window-normalize-window window)
+    (when git-gutter-mode
+      (git-gutter))))
 
 (defsubst git-gutter:diff-process-buffer (curfile)
   (concat " *git-gutter-" curfile "-*"))
@@ -607,8 +578,8 @@ Argument TEST is the case before BODY execution."
             (make-local-variable 'git-gutter:diffinfos)
             ;;(setq-local git-gutter:start-revision nil)
             (add-hook 'kill-buffer-hook 'git-gutter:kill-buffer-hook nil t)
-            (add-hook 'pre-command-hook 'git-gutter:pre-command-hook t)
-            (add-hook 'post-command-hook 'git-gutter:post-command-hook nil t)
+            (add-hook 'window-buffer-change-functions
+                      #'git-gutter:window-buffer-change-function nil t)
             (dolist (hook git-gutter:update-hooks)
               (add-hook hook 'git-gutter nil t))
             (git-gutter)
@@ -621,10 +592,10 @@ Argument TEST is the case before BODY execution."
           (message "Here is not %s work tree" (git-gutter:show-backends)))
         (git-gutter-mode -1))
     (remove-hook 'kill-buffer-hook 'git-gutter:kill-buffer-hook t)
-    (remove-hook 'pre-command-hook 'git-gutter:pre-command-hook t)
-    (remove-hook 'post-command-hook 'git-gutter:post-command-hook t)
     (dolist (hook git-gutter:update-hooks)
       (remove-hook hook 'git-gutter t))
+    (remove-hook 'window-buffer-change-functions
+                 #'git-gutter:window-buffer-change-function t)
     (git-gutter:clear-gutter)))
 
 (defun git-gutter--turn-on ()
@@ -980,19 +951,6 @@ Argument TEST is the case before BODY execution."
     (run-with-idle-timer 0.1 nil 'git-gutter)))
 (advice-add 'toggle-truncate-lines :after #'git-gutter:toggle-truncate-lines)
 
-;; `quit-window' and `switch-to-buffer' are called from other
-;; commands. So calling git-gutter from `post-command-hook' is not enough, use
-;; advices instead.
-(defun git-gutter:quit-window (&rest _args)
-  (when git-gutter-mode
-    (git-gutter)))
-(advice-add 'quit-window :after #'git-gutter:quit-window)
-
-(defun git-gutter:switch-to-buffer (&rest _args)
-  (when git-gutter-mode
-    (git-gutter)))
-(advice-add 'switch-to-buffer :after #'git-gutter:switch-to-buffer)
-
 (defun git-gutter:clear ()
   "Clear diff information in gutter."
   (interactive)
@@ -1178,6 +1136,7 @@ start revision."
                    (- (git-gutter-hunk-end-line hunk)
                       (git-gutter-hunk-start-line hunk))))))
 
+# FIXME: This does not work as advertised.
 (defun git-gutter:statistic ()
   "Return statistic unstaged hunks in current buffer."
   (interactive)