emacs-28 0c51db6: Amend Follow Mode to handle header lines and tab lines correctly

[Alan Mackenzie]

branch: emacs-28
commit 0c51db6bb19c0142fc0d866a7257e21de83374f3
Author: Alan Mackenzie <acm@muc.de>
Commit: Alan Mackenzie <acm@muc.de>

    Amend Follow Mode to handle header lines and tab lines correctly
    
    This fixes bug #51590.
    
    list/follow.el (follow-scroll-down): Incorporate the height of the tab line
    into the calculation of the window height.
    (follow-calc-win-end): Incorporate the pixel heights of the header line and
    the tab line the calculation of the buffer position of the bottom screen 
line.
---
 lisp/follow.el | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/lisp/follow.el b/lisp/follow.el
index b64f4cb..2ca2c1f 100644
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -667,7 +667,8 @@ Works like `scroll-down' when not in Follow mode."
         (scroll-down-command arg))
        (arg (follow-scroll-down-arg arg))
         (t
-        (let* ((windows (follow-all-followers))
+        (let* ((orig-point (point))
+                (windows (follow-all-followers))
                (win (car (reverse windows)))
                (start (window-start (car windows))))
           (if (eq start (point-min))
@@ -678,11 +679,14 @@ Works like `scroll-down' when not in Follow mode."
             (select-window win)
             (goto-char start)
             (vertical-motion (- (- (window-height win)
-                                   (if header-line-format 2 1)
-                                   next-screen-context-lines)))
+                                   (if header-line-format 2 1) ; always 
mode-line
+                                   (if tab-line-format 1 0)
+                                    next-screen-context-lines)))
             (set-window-start win (point))
-            (goto-char start)
-            (vertical-motion (- next-screen-context-lines 1))
+             (if (< orig-point (window-end win t))
+                 (goto-char orig-point)
+               (goto-char start)
+              (vertical-motion (- next-screen-context-lines 1)))
             (setq follow-internal-force-redisplay t))))))
 (put 'follow-scroll-down 'scroll-command t)
 
@@ -947,7 +951,11 @@ used."
   (let* ((win (or win (selected-window)))
         (edges (window-inside-pixel-edges win))
         (ht (- (nth 3 edges) (nth 1 edges)))
-        (last-line-pos (posn-point (posn-at-x-y 0 (1- ht) win))))
+        (last-line-pos (posn-point
+                         (posn-at-x-y 0 (+ (window-header-line-height win)
+                                           (window-tab-line-height win)
+                                           (1- ht))
+                                      win))))
     (if (pos-visible-in-window-p last-line-pos win)
        (let ((end (window-end win t)))
          (list end (pos-visible-in-window-p (point-max) win)))