Subject: bug#50220: 27.2; follow.el: follow-recenter does not work correctly if invoked in a follow window before the middle follow window.

From: Noah Evans <[email-masked]>
Date: Fri, 27 Aug 2021 00:39:26 +0000

In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Arch Linux

In follow mode, 'follow-mode-recenter` with no argument should center
the middle window around point. It works fine if it is used in the
middle window or a window after it, but if it is used in a window before
middle the post-command-hook moves the window back.

Reproduce:
Find a long file.
Split the window vertically two or more times.
M-x follow-mode
Go to the leftmost window and do C-c . C-l

Patch to fix:

From 65c6ccae67a5b2744711c3bfae2ebff3e530c2c1 Mon Sep 17 00:00:00 2001
From: Noah Evans <[email-masked]>
Date: Thu, 26 Aug 2021 20:27:45 -0400
Subject: [PATCH] follow.el: Fix bug#XXXXX in follow-recenter.

When follow-recenter is called in a follow window before the middle
follow window, the post-command-hook undoes the recentering.

* list/follow.el (follow-recenter): Set
follow-internal-force-redisplay when dest point is before start of
middle window.
---
lisp/follow.el | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/lisp/follow.el b/lisp/follow.el
index dde140d0fd5..b64f4cb7348 100644
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -858,8 +858,11 @@ from the bottom."
(windows (follow-all-followers))
(win (nth (/ (- (length windows) 1) 2) windows)))
(select-window win)
- (goto-char dest)
- (recenter))))
+ (let ((win-s (window-start)))
+ (goto-char dest)
+ (recenter)
+ (when (< dest win-s)
+ (setq follow-internal-force-redisplay t))))))

(defun follow-redraw ()
--
2.33.0