GNU bug report logs - #51814
follow-scroll-down leaves point in wrong place in a corner case.

Previous Next

Full log

View this message in rfc822 format

From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Alan Mackenzie <acm <at> muc.de>
Subject: bug#51814: closed (Re: bug#51814: follow-scroll-down leaves point
 in wrong place in a corner case.)
Date: Sat, 13 Nov 2021 18:43:02 +0000

[Message part 1 (text/plain, inline)]

Your bug report

#51814: follow-scroll-down leaves point in wrong place in a corner case.

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 51814 <at> debbugs.gnu.org.

-- 
51814: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=51814
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: Alan Mackenzie <acm <at> muc.de>
To: Juri Linkov <juri <at> linkov.net>
Cc: 51814-done <at> debbugs.gnu.org
Subject: Re: bug#51814: follow-scroll-down leaves point in wrong place in a
 corner case.
Date: Sat, 13 Nov 2021 18:42:29 +0000

Hello, Juri

On Sat, Nov 13, 2021 at 19:58:12 +0200, Juri Linkov wrote:
> > I believe the following patch fixes this.  Juri, could you try it out,
> > please.

> > As a test buffer,
> > (i) open elisp.info in a default GUI window (34 lines);
> > (ii) C-x 3 ; split it with a vertical divider.
> > (iii) ] ; Move to next page in manual, "Introduction".
> > (iv) M-x follow-mode.
> > (v) C-u 20 C-v ; scroll-up a significant amount.

> > (vi) Move point to the line beginning "* Caveats".
> > (vii) M-x follow-scroll-down.

> > Note that point is in a random place.  This is a bug.

> Thanks, I confirm this bug is fixed with your patch.
> I've tested it with and without tab-line-mode in Info
> with the header line.

Thanks for the testing!  I've committed the patch to the emacs-28
branch, and I'm closing the bug with this post.

-- 
Alan Mackenzie (Nuremberg, Germany).

[Message part 3 (message/rfc822, inline)]

From: Alan Mackenzie <acm <at> muc.de>
To: bug-gnu-emacs <at> gnu.org, Juri Linkov <juri <at> linkov.net>
Cc: acm <at> muc.de
Subject: follow-scroll-down leaves point in wrong place in a corner case.
Date: Sat, 13 Nov 2021 17:46:18 +0000

Hello, Juri and Emacs.

On the emacs-28 branch and master.

This bug is a corner case, noted but not fixed in bug #51590.

From a post in that bug thread:

[*] There is a situation which is not new, where if the buffer is too
short to fill all the windows, and it is already scrolled up far,
follow-scroll-down will scroll a correct amount, but leave point in a
random position.

I believe the following patch fixes this.  Juri, could you try it out,
please.

As a test buffer,
(i) open elisp.info in a default GUI window (34 lines);
(ii) C-x 3 ; split it with a vertical divider.
(iii) ] ; Move to next page in manual, "Introduction".
(iv) M-x follow-mode.
(v) C-u 20 C-v ; scroll-up a significant amount.

(vi) Move point to the line beginning "* Caveats".
(vii) M-x follow-scroll-down.

Note that point is in a random place.  This is a bug.

The cause of the bug is that the current code is "optimised" for the case
where there are enough buffer lines above point to scroll a full
follow-page, yet neglects the case where this doesn't hold.  The patch
does away with this optimisation.


diff --git a/lisp/follow.el b/lisp/follow.el
index 2ca2c1f17b..3761275bbf 100644
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -669,24 +669,30 @@ follow-scroll-down
         (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)
 

-- 
Alan Mackenzie (Nuremberg, Germany).

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.