bug#64855: 30.0.50; ERC 5.6: Make scrolltobottom less erratic

["J.P." <jp@neverwas.me>]

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Tags: patch

A minor bug fix involving the `scrolltobottom' module (and originally
lumped in with the mostly unrelated bug#63595) eventually materialized
on HEAD as [1]. While it did fix the bug, it didn't really address any
of the related foundational issues that have haunted `scrolltobottom'
since the beginning. The attached patch is meant to make the module more
predictable and consistent overall, while specifically addressing

  - the effect only being applied in the selected window

  - the effect not responding to changes in window size

  - the gradual scrolling of point toward the centermost line when
    away from the prompt (on graphical displays)

People miffed by this module's rough edges are encouraged to try these
changes and give feedback.

Thanks.


[1] commit e51e43b7046b56c58310854182a1d589ee4c770c
    Author: F. Jason Park <jp@neverwas.me>
    Date:   Wed May 17 19:48:02 2023 -0700

      Fix buffer-mismatch bug in erc-scroll-to-bottom

      * lisp/erc/erc-goodies.el (erc-scroll-to-bottom): Only
      `recenter' when the selected window's buffer is current.
      Previously, the module `scrolltobottom' signaled an "Error in
      `post-command-hook'" when a user clicked a channel indicator in
      the mode line from a window showing another ERC buffer.
    [...]
      (Bug#63595)

    lisp/erc/erc-goodies.el | 1 +
    lisp/erc/erc-track.el   | 9 ++++++++-
    2 files changed, 9 insertions(+), 1 deletion(-)


In GNU Emacs 30.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
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Repository revision: 89558533683a100ca7946c4a35bf4ef50463efef
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Fedora Linux 37 (Workstation Edition)

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From 5ce39aa8adfefc85208448ff1d254766f15892e1 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <jp@neverwas.me>
Date: Sat, 22 Jul 2023 00:46:44 -0700
Subject: [PATCH] [5.6] Consider all windows in erc-scrolltobottom-mode

* lisp/erc/erc-goodies.el (erc-scrolltobottom-mode,
erc-scrolltobottom-enable, erc-scrolltobottom-disable): Use
`erc--scroll-to-bottom-all' instead of `erc-possibly-scroll-to-bottom'
for `erc-insert-done-hook' and now also `erc-send-completed-hook'.
Call `erc-add-scroll-to-bottom' for teardown as well.
(erc--scroll-to-bottom-debounce-expire): New variable.
(erc-possibly-scroll-to-bottom): Don't bother scrolling when user has
done so recently.
(erc--scroll-to-bottom-all): New function to scroll in all windows of
the current buffer.
(erc-add-scroll-to-bottom): Perform teardown as well when mode is
disabled.  Also run on `window-configuration-changed-hook'.
(erc-scroll-to-bottom): Use `window-point' instead of `point'.
---
 lisp/erc/erc-goodies.el | 81 +++++++++++++++++++++++++++++++++++------
 1 file changed, 70 insertions(+), 11 deletions(-)

diff --git a/lisp/erc/erc-goodies.el b/lisp/erc/erc-goodies.el
index d9ededa8e68..af44b98e0bf 100644
--- a/lisp/erc/erc-goodies.el
+++ b/lisp/erc/erc-goodies.el
@@ -52,34 +52,93 @@ erc-input-line-position
 (define-erc-module scrolltobottom nil
   "This mode causes the prompt to stay at the end of the window."
   ((add-hook 'erc-mode-hook #'erc-add-scroll-to-bottom)
-   (add-hook 'erc-insert-done-hook #'erc-possibly-scroll-to-bottom)
+   (add-hook 'erc-insert-pre-hook #'erc--on-pre-insert)
+   (add-hook 'erc-insert-done-hook #'erc--scroll-to-bottom-all)
+   (add-hook 'erc-send-completed-hook #'erc--scroll-to-bottom-all)
    (unless erc--updating-modules-p (erc-buffer-do #'erc-add-scroll-to-bottom)))
   ((remove-hook 'erc-mode-hook #'erc-add-scroll-to-bottom)
-   (remove-hook 'erc-insert-done-hook #'erc-possibly-scroll-to-bottom)
-   (dolist (buffer (erc-buffer-list))
-     (with-current-buffer buffer
-       (remove-hook 'post-command-hook #'erc-scroll-to-bottom t)))))
+   (remove-hook 'erc-insert-pre-hook #'erc--on-pre-insert)
+   (remove-hook 'erc-insert-done-hook #'erc--scroll-to-bottom-all)
+   (remove-hook 'erc-send-completed-hook #'erc--scroll-to-bottom-all)
+   (erc-buffer-do #'erc-add-scroll-to-bottom)))
+
+(defvar-local erc--scroll-to-bottom-debounce-expire nil
+  "Time after which `scrolltobottom' is allowed to run.
+Set to a fraction of a second in the future on every refresh.")
+
+(defvar-local erc--scroll-to-bottom-last-window-start nil
+  "A cons of a window and a starting position.")
 
 (defun erc-possibly-scroll-to-bottom ()
   "Like `erc-add-scroll-to-bottom', but only if window is selected."
   (when (eq (selected-window) (get-buffer-window))
     (erc-scroll-to-bottom)))
 
+(defun erc--possibly-scroll-to-bottom ()
+  "Call `erc-scroll-to-bottom' when buffer occupies selected window.
+Skip when `erc--scroll-to-bottom-debounce-expire' has not yet
+arrived."
+  (when (eq (selected-window) (get-buffer-window))
+    (unless (eq this-command 'recenter-top-bottom)
+      (let (erc--scroll-to-bottom-last-window-start)
+        (erc--scroll-to-bottom)))))
+
+(defun erc--scroll-to-bottom-all (&rest _)
+  "Run `erc-scroll-to-bottom' in all windows showing current buffer."
+  (dolist (window (get-buffer-window-list nil nil 'visible))
+    (with-selected-window window
+      (erc--scroll-to-bottom))))
+
 (defun erc-add-scroll-to-bottom ()
-  "A hook function for `erc-mode-hook' to recenter output at bottom of window.
+  "Arrange for `scrolltobottom' to refresh on window configuration changes.
+Undo that arrangement when `erc-scrolltobottom-mode' is disabled.
 
 If you find that ERC hangs when using this function, try customizing
 the value of `erc-input-line-position'.
 
-This works whenever scrolling happens, so it's added to
-`window-scroll-functions' rather than `erc-insert-post-hook'."
-  (add-hook 'post-command-hook #'erc-scroll-to-bottom nil t))
+Note that the prior suggestion comes from a time when this
+function used `window-scroll-functions', which was replaced by
+`post-command-hook' in ERC 5.3."
+  (if erc-scrolltobottom-mode
+      (progn
+        (add-hook 'window-configuration-change-hook
+                  #'erc--possibly-scroll-to-bottom nil t)
+        (add-hook 'post-command-hook
+                  #'erc--possibly-scroll-to-bottom nil t))
+    (remove-hook 'window-configuration-change-hook
+                 #'erc--possibly-scroll-to-bottom t)
+    (remove-hook 'post-command-hook
+                 #'erc--possibly-scroll-to-bottom t)))
+
+(defun erc--on-pre-insert (&rest _)
+  (when (eq (selected-window) (get-buffer-window))
+    (setq erc--scroll-to-bottom-last-window-start
+          (cons (selected-window) (window-start)))))
+
+(defun erc--scroll-to-bottom ()
+  "Like `erc-scroll-to-bottom', but use `window-point'.
+Expect to run in some window, not necessarily the user-selected
+one."
+  (when erc-insert-marker
+    (let ((resize-mini-windows nil))
+      (save-restriction
+        (widen)
+        (if (>= (window-point) erc-input-marker)
+            (save-excursion
+              (goto-char (point-max))
+              (recenter (or erc-input-line-position -1)))
+          (when (and erc--scroll-to-bottom-last-window-start
+                     ;; `selected-window' means the one being visited.
+                     (eq (selected-window)
+                         (car erc--scroll-to-bottom-last-window-start)))
+            (save-excursion
+              (goto-char (cdr erc--scroll-to-bottom-last-window-start))
+              (let ((recenter-positions '(top)))
+                (recenter-top-bottom)))))))))
 
 (defun erc-scroll-to-bottom ()
   "Recenter WINDOW so that `point' is on the last line.
 
-This is added to `window-scroll-functions' by `erc-add-scroll-to-bottom'.
-
 You can control which line is recentered to by customizing the
 variable `erc-input-line-position'."
       ;; Temporarily bind resize-mini-windows to nil so that users who have it
-- 
2.41.0