emacs-28 d4536ff: Fix follow-scroll-down in a small buffer which starts slightly scrolled

[Alan Mackenzie]

branch: emacs-28
commit d4536ff2572931b105198a85a452a777d6d3a1ff
Author: Alan Mackenzie <acm@muc.de>
Commit: Alan Mackenzie <acm@muc.de>

    Fix follow-scroll-down in a small buffer which starts slightly scrolled
    
    This fixes bug #51814.
    
    * lisp/follow.el (follow-scroll-down): Do away with the optimization of 
doing
    vertical-motion over only one window.  Instead move over all windows, to
    checck for being close to point-min, and setting point accordingly.
---
 lisp/follow.el | 32 +++++++++++++++++++-------------
 1 file changed, 19 insertions(+), 13 deletions(-)

diff --git a/lisp/follow.el b/lisp/follow.el
index 2ca2c1f..3761275 100644
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -669,24 +669,30 @@ Works like `scroll-down' when not in Follow mode."
         (t
         (let* ((orig-point (point))
                 (windows (follow-all-followers))
-               (win (car (reverse windows)))
-               (start (window-start (car windows))))
+               (start (window-start (car windows)))
+                (lines 0))
           (if (eq start (point-min))
               (if (or (null scroll-error-top-bottom)
                       (bobp))
                   (signal 'beginning-of-buffer nil)
                 (goto-char (point-min)))
-            (select-window win)
-            (goto-char start)
-            (vertical-motion (- (- (window-height win)
-                                   (if header-line-format 2 1) ; always 
mode-line
-                                   (if tab-line-format 1 0)
-                                    next-screen-context-lines)))
-            (set-window-start win (point))
-             (if (< orig-point (window-end win t))
-                 (goto-char orig-point)
-               (goto-char start)
-              (vertical-motion (- next-screen-context-lines 1)))
+             (select-window (car windows))
+             (dolist (win windows)
+               (setq lines
+                     (+ lines
+                        (- (window-height win)
+                           (if header-line-format 2 1) ; Count mode-line, too.
+                           (if tab-line-format 1 0)))))
+             (setq lines (- lines next-screen-context-lines))
+             (goto-char start)
+             (let ((at-top (> (vertical-motion (- lines)) (- lines))))
+               (set-window-start (car windows) (point))
+               (if at-top
+                   (goto-char orig-point)
+                 (goto-char start)
+                 (vertical-motion (- next-screen-context-lines 1))
+                 (if (< orig-point (point))
+                     (goto-char orig-point))))
             (setq follow-internal-force-redisplay t))))))
 (put 'follow-scroll-down 'scroll-command t)