(scroll_preserve_screen_position): New variable.

(keys_of_window): Set up Lisp var.
(window_scroll): If scroll_preserve_screen_position is 0,
behave the old way (advancing point the minimum distance).

diff --git a/src/window.c b/src/window.c
index dd5d472..071c3dc 100644 (file)
--- a/src/window.c
+++ b/src/window.c
@@ -111,8 +111,14 @@ static int sequence_number;
 /* Nonzero after init_window_once has finished.  */
 static int window_initialized;
 
+/* Nonzero means scroll commands try to put point
+   at the same screen height as previously.  */
+static int scroll_preserve_screen_position;
+
 #define min(a, b) ((a) < (b) ? (a) : (b))
 
+extern int scroll_margin;
+
 extern Lisp_Object Qwindow_scroll_functions, Vwindow_scroll_functions;
 \f
 DEFUN ("windowp", Fwindowp, Swindowp, 1, 1, 0,
@@ -2638,8 +2644,7 @@ window_internal_width (w)
 
 
 /* Scroll contents of window WINDOW up N lines.
-   If WHOLE is nonzero, it means we wanted to scroll
-   by entire screenfuls.  */
+   If WHOLE is nonzero, it means scroll N screenfuls instead.  */
 
 static void
 window_scroll (window, n, whole, noerror)
@@ -2693,8 +2698,6 @@ window_scroll (window, n, whole, noerror)
 
   if (pos < ZV)
     {
-      extern int scroll_margin;
-
       int this_scroll_margin = scroll_margin;
 
       /* Don't use a scroll margin that is negative or too large.  */
@@ -2713,7 +2716,7 @@ window_scroll (window, n, whole, noerror)
         the window-scroll-functions.  */
       w->force_start = Qt;
 
-      if (whole)
+      if (whole && scroll_preserve_screen_position)
        {
          SET_PT (pos);
          Fvertical_motion (make_number (original_vpos), window);
@@ -2735,11 +2738,13 @@ window_scroll (window, n, whole, noerror)
 
          if (top_margin <= opoint)
            SET_PT (opoint);
-         else
+         else if (scroll_preserve_screen_position)
            {
              SET_PT (pos);
              Fvertical_motion (make_number (original_vpos), window);
            }
+         else
+           SET_PT (pos);
        }
       else if (n < 0)
        {
@@ -2758,8 +2763,13 @@ window_scroll (window, n, whole, noerror)
            SET_PT (opoint);
          else
            {
-             SET_PT (pos);
-             Fvertical_motion (make_number (original_vpos), window);
+             if (scroll_preserve_screen_position)
+               {
+                 SET_PT (pos);
+                 Fvertical_motion (make_number (original_vpos), window);
+               }
+             else
+               Fvertical_motion (make_number (-1), window);
            }
        }
     }
@@ -3644,6 +3654,11 @@ If there is only one window, it is split regardless of this value.");
     "*Delete any window less than this wide.");
   window_min_width = 10;
 
+  DEFVAR_BOOL ("scroll-preserve-screen-position",
+              &scroll_preserve_screen_position,
+    "*Nonzero means scroll commands move point to keep its screen line unchanged.");
+  scroll_preserve_screen_position = 0;
+
   defsubr (&Sselected_window);
   defsubr (&Sminibuffer_window);
   defsubr (&Swindow_minibuffer_p);