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

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

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"J.P." <jp@neverwas.me> writes:

> I've added these changes as
>
>   https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=328a9856
>
> This bug has already been closed. Thanks.

But wait, there's more! (There's always more.)

It seems I went a little overboard in reigning in the frequency of
scroll attempts. This has been walked back slightly in the attached
changes.

There's also a subtle quirk that's come to light involving `point-max'
and (recenter -1) where Emacs calculates point as having drifted off
screen. We could try tackling this with something kludgey, like
decrementing point temporarily during `recenter' attempts. But that
fails if the current input ends in a newline. I've instead decided to
address this by taking a page from other Emacs libraries and setting
`scroll-step' locally to 1 if a user hasn't customized
`scroll-conservatively'. See attached.


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From 3e538317bade00780bb54a31aafd58689ee9eb42 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <jp@neverwas.me>
Date: Tue, 24 Oct 2023 18:18:50 -0700
Subject: [PATCH] [5.6] Be slightly more aggressive in erc-scrolltobottom-all

* lisp/erc/erc-goodies.el (erc--scrolltobottom-on-post-command):
Redo obsolete doc string.
(erc--scrolltobottom-at-prompt-minibuffer-active,
erc--scrolltobottom-on-win-conf-change): Rename former to latter to
better reflect actual role.  Remove conditional guard so this always
runs.
(erc--scrolltobottom-setup): Set `scroll-step' locally when a user
hasn't customized `scroll-conservatively'.  Update hook member names.
(Bug#64855)
---
 lisp/erc/erc-goodies.el | 26 ++++++++++----------------
 1 file changed, 10 insertions(+), 16 deletions(-)

diff --git a/lisp/erc/erc-goodies.el b/lisp/erc/erc-goodies.el
index 9d70c644429..e854d063cdd 100644
--- a/lisp/erc/erc-goodies.el
+++ b/lisp/erc/erc-goodies.el
@@ -119,28 +119,20 @@ erc--scrolltobottom-post-ignore-commands
   "Commands to skip instead of force-scroll on `post-command-hook'.")
 
 (defun erc--scrolltobottom-on-post-command ()
-  "Restore window start or scroll to prompt and recenter.
-When `erc--scrolltobottom-window-info' is non-nil and its first
-item is associated with the selected window, restore start of
-window so long as prompt hasn't moved.  Expect buffer to be
-unnarrowed."
+  "Scroll selected window unless `this-command' is exempted."
   (when (eq (selected-window) (get-buffer-window))
     (unless (memq this-command erc--scrolltobottom-post-ignore-commands)
-      (erc--scrolltobottom-confirm))
-    (setq erc--scrolltobottom-window-info nil)))
+      (setq erc--scrolltobottom-window-info nil)
+      (erc--scrolltobottom-confirm))))
 
 ;; It may be desirable to also restore the relative line position of
 ;; window point after changing dimensions.  Perhaps stashing the
 ;; previous ratio of window line to body height and later recentering
 ;; proportionally would achieve this.
-(defun erc--scrolltobottom-at-prompt-minibuffer-active ()
+(defun erc--scrolltobottom-on-win-conf-change ()
   "Scroll window to bottom when at prompt and using the minibuffer."
-  ;; This is redundant or ineffective in the selected window if at
-  ;; prompt or if only one window exists.
-  (unless (or (input-pending-p)
-              (and (minibuffer-window-active-p (minibuffer-window))
-                   (eq (old-selected-window) (minibuffer-window))))
-    (erc--scrolltobottom-confirm)))
+  (setq erc--scrolltobottom-window-info nil)
+  (erc--scrolltobottom-confirm))
 
 (defun erc--scrolltobottom-all (&rest _)
   "Maybe put prompt on last line in all windows displaying current buffer.
@@ -176,16 +168,18 @@ erc--scrolltobottom-setup
       (if erc-scrolltobottom-all
           (progn
             (setq-local read-minibuffer-restore-windows nil)
+            (when (zerop scroll-conservatively)
+              (setq-local scroll-step 1))
             (unless (eq erc-scrolltobottom-all 'relaxed)
               (add-hook 'window-configuration-change-hook
-                        #'erc--scrolltobottom-at-prompt-minibuffer-active 50 t)
+                        #'erc--scrolltobottom-on-win-conf-change 50 t)
               (add-hook 'post-command-hook
                         #'erc--scrolltobottom-on-post-command 50 t)))
         (add-hook 'post-command-hook #'erc-scroll-to-bottom nil t))
     (remove-hook 'post-command-hook #'erc-scroll-to-bottom t)
     (remove-hook 'post-command-hook #'erc--scrolltobottom-on-post-command t)
     (remove-hook 'window-configuration-change-hook
-                 #'erc--scrolltobottom-at-prompt-minibuffer-active t)
+                 #'erc--scrolltobottom-on-win-conf-change t)
     (kill-local-variable 'read-minibuffer-restore-windows)
     (kill-local-variable 'erc--scrolltobottom-window-info)))
 
-- 
2.41.0