--- Begin Message ---
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Subject: |
follow-scroll-down leaves point in wrong place in a corner case. |
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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).
--- End Message ---
--- Begin Message ---
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Subject: |
Re: bug#51814: follow-scroll-down leaves point in wrong place in a corner case. |
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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).
--- End Message ---