bug#51590: follow-mode is broken with header-line and tab-line

[Juri Linkov]

retitle 51590 follow-mode is broken with header-line and tab-line
quit

>> The most low-level function of follow-mode is follow-calc-win-end.
>> When global-tab-line-mode is enabled, follow-calc-win-end
>> returns the same values as when global-tab-line-mode is disabled.
>
> Can you step through that function and see which part is misbehaving
> there?
>
>> I don't understand what more low-level function doesn't take
>> into account the height of the tab-line.  Maybe the problem is
>> in pos-visible-in-window-p?  Or maybe different values returned
>> by window-inside-pixel-edges and window-end?
>
> I don't know the answer, but I see that header-line-format is
> mentioned in one place in follow.el, and that is not a good sign...

Indeed, I tested with the header-line, and it has the same problem,
so retitled this bug report.

After trial and error, I arrived to the following patch that completely
fixes these problems.  However, I can't explain how it works.

diff --git a/lisp/follow.el b/lisp/follow.el
index b64f4cb734..327c66d5ff 100644
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -679,6 +679,7 @@ follow-scroll-down
             (goto-char start)
             (vertical-motion (- (- (window-height win)
                                    (if header-line-format 2 1)
+                                   (if tab-line-format 2 1)
                                    next-screen-context-lines)))
             (set-window-start win (point))
             (goto-char start)
@@ -946,8 +947,10 @@ follow-calc-win-end
 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))))
+        (ht (+ (- (nth 3 edges) (nth 1 edges))
+               (window-header-line-height win)
+               (window-tab-line-height win)))
+        (last-line-pos (posn-point (posn-at-x-y 0 (- ht 1) 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)))