bug#12403: 24.2; scroll-margin breaks emacs24

bug#12403: 24.2; scroll-margin breaks emacs24

[Yuri D'Elia]

emacs24 as shipped by debian breaks "scroll-margin".

   http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=681991

It seems that setting scroll-margin on *any* value breaks the redisplay
while scrolling.

Try with the following (under a graphical session):

emacs -q

open the tutorial, scroll with the arrow keys until you reach the bottom.
Everything looks ok.

kill emacs. Start again, this time:

C-x set-variable scroll-margin 5

open the tutorial, scroll down with the arrow keys until you reach the
bottom.  Text on the page looks *badly* garbled (this is not just
character left-overs from the previous screenfull, a lot of lines look
repeated at some point in the same line).


In GNU Emacs 24.2.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.10)
  of 2012-09-09 on trouble, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11203902
Configured using:
  `configure '--build' 'x86_64-linux-gnu' '--build' 'x86_64-linux-gnu'
  '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
  '--localstatedir=/var/lib' '--infodir=/usr/share/info'
  '--mandir=/usr/share/man' '--with-pop=yes'
 
'--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.2/site-lisp:/usr/share/emacs/site-lisp'
  '--with-crt-dir=/usr/lib/x86_64-linux-gnu' '--with-x=yes'
  '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars'
  'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
  --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
  'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
   value of $LC_ALL: nil
   value of $LC_COLLATE: C
   value of $LC_CTYPE: nil
   value of $LC_MESSAGES: nil
   value of $LC_MONETARY: nil
   value of $LC_NUMERIC: nil
   value of $LC_TIME: nil
   value of $LANG: en_US.UTF-8
   value of $XMODIFIERS: nil
   locale-coding-system: utf-8-unix
   default enable-multibyte-characters: t

Major mode: Fundamental

Minor modes in effect:
   tooltip-mode: t
   mouse-wheel-mode: t
   tool-bar-mode: t
   menu-bar-mode: t
   file-name-shadow-mode: t
   global-font-lock-mode: t
   blink-cursor-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   line-number-mode: t
   transient-mark-mode: t

Recent messages:
Loading /etc/emacs/site-start.d/50php-elisp.el (source)...done
Loading /etc/emacs/site-start.d/50psvn.el (source)...done
Loading /etc/emacs/site-start.d/50pymacs.el (source)...done
Loading /etc/emacs/site-start.d/50python-docutils.el (source)...done
Loading /etc/emacs/site-start.d/50python-mode.el (source)...done
Loading /etc/emacs/site-start.d/50w3m-el.el (source)...done
Loading /etc/emacs/site-start.d/50yaml-mode.el (source)...done
Loading /etc/emacs/site-start.d/51debian-el.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Load-path shadows:
/usr/share/emacs/24.2/site-lisp/cmake-data/cmake-mode hides 
/usr/share/emacs/site-lisp/cmake-mode
/usr/share/emacs24/site-lisp/dictionaries-common/ispell hides 
/usr/share/emacs/24.2/lisp/textmodes/ispell
/usr/share/emacs24/site-lisp/dictionaries-common/flyspell hides 
/usr/share/emacs/24.2/lisp/textmodes/flyspell
/usr/share/emacs/site-lisp/rst hides 
/usr/share/emacs/24.2/lisp/textmodes/rst

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail regexp-opt rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils help-mode easymenu view debian-el
debian-el-loaddefs w3m-load mmm-auto mmm-vars mmm-compat cl
emacs-goodies-el emacs-goodies-custom emacs-goodies-loaddefs easy-mmode
time-date tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
tool-bar dnd fontset image fringe lisp-mode register page menu-bar
rfn-eshadow timer select scroll-bar mouse jit-lock font-lock syntax
facemenu font-core frame cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese hebrew greek romanian slovak
czech european ethiopic indian cyrillic chinese case-table epa-hook
jka-cmpr-hook help simple abbrev minibuffer loaddefs button faces
cus-face files text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget hashtable-print-readable backquote
make-network-process dbusbind dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)

bug#12403: 24.2; scroll-margin breaks emacs24

[Eli Zaretskii]

> Date: Mon, 10 Sep 2012 16:50:22 +0200
> From: Yuri D'Elia <wavexx@thregr.org>
> 
> emacs24 as shipped by debian breaks "scroll-margin".
> 
>    http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=681991
> 
> It seems that setting scroll-margin on *any* value breaks the redisplay
> while scrolling.
> 
> Try with the following (under a graphical session):
> 
> emacs -q
> 
> open the tutorial, scroll with the arrow keys until you reach the bottom.
> Everything looks ok.
> 
> kill emacs. Start again, this time:
> 
> C-x set-variable scroll-margin 5
> 
> open the tutorial, scroll down with the arrow keys until you reach the
> bottom.  Text on the page looks *badly* garbled (this is not just
> character left-overs from the previous screenfull, a lot of lines look
> repeated at some point in the same line).

Confirmed :-(.  However, this bug was present even in the first
pretest of Emacs 24, 24.0.90.

I will take a look.

bug#12403: 24.2; scroll-margin breaks emacs24

[Eli Zaretskii]

> Date: Mon, 10 Sep 2012 19:39:41 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 12403@debbugs.gnu.org
> 
> > Try with the following (under a graphical session):
> > 
> > emacs -q
> > 
> > open the tutorial, scroll with the arrow keys until you reach the bottom.
> > Everything looks ok.
> > 
> > kill emacs. Start again, this time:
> > 
> > C-x set-variable scroll-margin 5
> > 
> > open the tutorial, scroll down with the arrow keys until you reach the
> > bottom.  Text on the page looks *badly* garbled (this is not just
> > character left-overs from the previous screenfull, a lot of lines look
> > repeated at some point in the same line).
> 
> Confirmed :-(.  However, this bug was present even in the first
> pretest of Emacs 24, 24.0.90.
> 
> I will take a look.

Fixed in revision 108138 on the emacs-24 branch, with the following
patch:

=== modified file 'src/ChangeLog'
--- src/ChangeLog	2012-09-12 14:59:56 +0000
+++ src/ChangeLog	2012-09-12 17:10:02 +0000
@@ -1,3 +1,11 @@
+2012-09-12  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (try_window_reusing_current_matrix): Under bidi
+	reordering, locate the cursor by calling set_cursor_from_row; if
+	that fails, clear the desired glyph matrix before returning a
+	failure indication to the caller.  Fixes leaving garbled display
+	when fast scrolling with a down-key.  (Bug#12403)
+
 2012-09-12  Jan Djärv  <jan.h.d@swipnet.se>
 
 	* gtkutil.c (x_wm_set_size_hint): Use 1 col for base_width so it

=== modified file 'src/xdisp.c'
--- src/xdisp.c	2012-05-30 18:09:17 +0000
+++ src/xdisp.c	2012-09-12 16:58:45 +0000
@@ -16593,28 +16593,33 @@ try_window_reusing_current_matrix (struc
 	    }
 	  if (row < bottom_row)
 	    {
-	      struct glyph *glyph = row->glyphs[TEXT_AREA] + w->cursor.hpos;
-	      struct glyph *end = row->glyphs[TEXT_AREA] + row->used[TEXT_AREA];
-
-	      /* Can't use this optimization with bidi-reordered glyph
-		 rows, unless cursor is already at point. */
+	      /* Can't simply scan the row for point with
+		 bidi-reordered glyph rows.  Let set_cursor_from_row
+		 figure out where to put the cursor, and if it fails,
+		 give up.  */
 	      if (!NILP (BVAR (XBUFFER (w->buffer), bidi_display_reordering)))
 		{
-		  if (!(w->cursor.hpos >= 0
-			&& w->cursor.hpos < row->used[TEXT_AREA]
-			&& BUFFERP (glyph->object)
-			&& glyph->charpos == PT))
-		    return 0;
+		  if (!set_cursor_from_row (w, row, w->current_matrix,
+					    0, 0, 0, 0))
+		    {
+		      clear_glyph_matrix (w->desired_matrix);
+		      return 0;
+		    }
 		}
 	      else
-		for (; glyph < end
-		       && (!BUFFERP (glyph->object)
-			   || glyph->charpos < PT);
-		     glyph++)
-		  {
-		    w->cursor.hpos++;
-		    w->cursor.x += glyph->pixel_width;
-		  }
+		{
+		  struct glyph *glyph = row->glyphs[TEXT_AREA] + w->cursor.hpos;
+		  struct glyph *end = row->glyphs[TEXT_AREA] + row->used[TEXT_AREA];
+
+		  for (; glyph < end
+			 && (!BUFFERP (glyph->object)
+			     || glyph->charpos < PT);
+		       glyph++)
+		    {
+		      w->cursor.hpos++;
+		      w->cursor.x += glyph->pixel_width;
+		    }
+		}
 	    }
 	}