GNU bug report logs - #19060
[FIX INCLUDED] Off-by-one-line scrolling bug in window_scroll_pixel_based

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#19060: closed ([FIX INCLUDED] Off-by-one-line scrolling bug
 in window_scroll_pixel_based)
Date: Sat, 15 Nov 2014 17:08:02 +0000

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Your message dated Sat, 15 Nov 2014 19:06:55 +0200
with message-id <831tp4fl5c.fsf <at> gnu.org>
and subject line Re: bug#19060: [FIX INCLUDED] Off-by-one-line scrolling bug in	window_scroll_pixel_based
has caused the debbugs.gnu.org bug report #19060,
regarding [FIX INCLUDED] Off-by-one-line scrolling bug in window_scroll_pixel_based
to be marked as done.

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From: Kelly Dean <kelly <at> prtime.org>
To: bug-gnu-emacs <at> gnu.org
Subject: [FIX INCLUDED] Off-by-one-line scrolling bug in
 window_scroll_pixel_based
Date: Sat, 15 Nov 2014 06:57:45 +0000

With 24.4, emacs -Q, then M-x split-window-horizontally, then open any file with a lot of long logical lines (each a few visual lines long), so that as you page down through the file, every screenful of text has some wrapped lines on it. Adjust the vertical size of your Emacs frame so that the vertical size of your Emacs windows is not an integer multiple of the visual line height, so that the bottom visual line of text is partially covered by the top of the mode line.
Scroll forward a couple pages using pgdn (scroll-up-command), then notice exactly which visual line is at the top, and which one is at the bottom. Press pgdn, then pgup, and notice that the text doesn't return to the previous position; instead, it's scrolled backward by one visual line. This bug wasn't in 24.3.

The bug is in window.c of the 24.4 release, line 5042. The fix is to change ⌜window_box_height (w)⌝ on that line to ⌜window_box_height (w) / dy * dy⌝ in order to align the result to an integer multiple of the visual line height, since as of 24.4, Emacs no longer ensures that window_box_height is such a multiple. Patch excluded, per request.

There might be a related bug (with the same fix) on line 4959, but I haven't tested that.

BTW on line 471 (copied from line 466) of window.h, ‟width” should be ‟height”.

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From: Eli Zaretskii <eliz <at> gnu.org>
To: martin rudalics <rudalics <at> gmx.at>
Cc: kelly <at> prtime.org, 19060-done <at> debbugs.gnu.org
Subject: Re: bug#19060: [FIX INCLUDED] Off-by-one-line scrolling bug
 in	window_scroll_pixel_based
Date: Sat, 15 Nov 2014 19:06:55 +0200

> Date: Sat, 15 Nov 2014 15:32:41 +0100
> From: martin rudalics <rudalics <at> gmx.at>
> CC: kelly <at> prtime.org, 19060 <at> debbugs.gnu.org
> 
>  > Since the issue is rounding vs truncating, perhaps try playing with
>  > the portion of the last displayed line, trying both less and greater
>  > than half the canonical line height.
> 
> Got it.  The last line must show more than a half of the frame's
> character height.  And the patch fixes it AFAICT.  So please apply it
> with as much comment as you can provide.

Done on the emacs-24 branch.

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