Re: The unwarranted scrolling assumption

[Eli Zaretskii <eliz@gnu.org>]

> From: Lennart Borgman <lennart.borgman@gmail.com>
> Date: Sat, 19 Jun 2010 15:00:37 +0200
> Cc: monnier@iro.umontreal.ca, cyd@stupidchicken.com, emacs-devel@gnu.org
> 
> On Sat, Jun 19, 2010 at 2:54 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> >
> > Update: with revno 100620, I can no longer get Emacs to recenter by
> > pressing and holding the <down> arrow key, unless bidi-display-reordering
> > is non-nil.  The latter is probably due to some bug in cursor positioning
> > in set_cursor_from_row and display_line, under bidirectional display; I
> > will look into that next.
> >
> > So please try the latest trunk and see if Emacs still recenters sometimes.
> 
> Can you please instead show your patch here?

Here:

=== modified file 'src/ChangeLog'
--- src/ChangeLog	2010-06-16 20:08:41 +0000
+++ src/ChangeLog	2010-06-19 09:39:37 +0000
@@ -1,3 +1,13 @@
+2010-06-19  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (try_scrolling): Compute the limit for searching point
+	in forward scroll from scroll_max, instead of an arbitrary limit
+	of 10 screen lines.  See
+	http://lists.gnu.org/archive/html/emacs-devel/2010-06/msg00766.html
+	and
+	http://lists.gnu.org/archive/html/emacs-devel/2010-06/msg00773.html
+	for details.
+
 2010-06-16  Glenn Morris  <rgm@gnu.org>
 
 	* editfns.c (Fbyte_to_string): Pacify compiler.

=== modified file 'src/xdisp.c'
--- src/xdisp.c	2010-06-01 02:34:49 +0000
+++ src/xdisp.c	2010-06-19 11:55:00 +0000
@@ -13431,14 +13431,22 @@ try_scrolling (window, just_this_one_p, 
       if (PT > CHARPOS (it.current.pos))
 	{
 	  int y0 = line_bottom_y (&it);
-
-	  /* Compute the distance from the scroll margin to PT
-	     (including the height of the cursor line).  Moving the
-	     iterator unconditionally to PT can be slow if PT is far
-	     away, so stop 10 lines past the window bottom (is there a
-	     way to do the right thing quickly?).  */
-	  move_it_to (&it, PT, -1,
-	  	      it.last_visible_y + 10 * FRAME_LINE_HEIGHT (f),
+	  /* Compute how many pixels below window bottom to stop searching
+	     for PT.  This avoids costly search for PT that is far away if
+	     the user limited scrolling by a small number of lines, but
+	     always finds PT if scroll_conservatively is set to a large
+	     number, such as most-positive-fixnum.  */
+	  int slack = max (scroll_max, 10 * FRAME_LINE_HEIGHT (f));
+	  int y_to_move =
+	    slack >= INT_MAX - it.last_visible_y
+	    ? INT_MAX
+	    : it.last_visible_y + slack;
+
+	  /* Compute the distance from the scroll margin to PT or to
+	     the scroll limit, whichever comes first.  This should
+	     include the height of the cursor line, to make that line
+	     fully visible.  */
+	  move_it_to (&it, PT, -1, y_to_move,
 	  	      -1, MOVE_TO_POS | MOVE_TO_Y);
 	  dy = line_bottom_y (&it) - y0;