Make limit on scroll-margin variable

* src/xdisp.c (maximum-scroll-margin): New variable.
* lisp/cus-start.el: Make it customizable.
* etc/NEWS: Mention it.
* doc/emacs/display.texi (Auto Scrolling):
* doc/lispref/windows.texi (Textual Scrolling): Document it.
* src/window.c (window_scroll_pixel_based): Use it instead of hardcoding
division by 4 (Bug #5718).

2 files changed, 22 insertions, 5 deletions

diff --git a/src/window.c b/src/window.c
index 235c3c1ade8..ba03780f3df 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4801,11 +4801,20 @@ window_scroll_margin (struct window *window, enum margin_unit unit)
     {
       int frame_line_height = default_line_pixel_height (window);
       int window_lines = window_box_height (window) / frame_line_height;
-      int margin = min (scroll_margin, window_lines / 4);
-      if (unit == MARGIN_IN_PIXELS)
-        return margin * frame_line_height;
-      else
-        return margin;
+
+      double ratio = 0.25;
+      if (FLOATP (Vmaximum_scroll_margin))
+        {
+          ratio = XFLOAT_DATA (Vmaximum_scroll_margin);
+          ratio = max (0.0, ratio);
+          ratio = min (ratio, 0.5);
+        }
+      int max_margin = min ((window_lines - 1)/2,
+                            (int) (window_lines * ratio));
+      int margin = clip_to_bounds (0, scroll_margin, max_margin);
+      return (unit == MARGIN_IN_PIXELS)
+        ? margin * frame_line_height
+        : margin;
     }
   else
     return 0;
diff --git a/src/xdisp.c b/src/xdisp.c
index 8a450b7a8a4..134ef6c6196 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -31520,6 +31520,14 @@ Recenter the window whenever point gets within this many lines
 of the top or bottom of the window.  */);
   scroll_margin = 0;
 
+  DEFVAR_LISP ("maximum-scroll-margin", Vmaximum_scroll_margin,
+    doc: /* Maximum effective value of `scroll-margin'.
+Given as a fraction of the current window's lines.  The value should
+be a floating point number between 0.0 and 0.5.  The effective maximum
+is limited to (/ (1- window-lines) 2).  Non-float values for this
+variable are ignored and the default 0.25 is used instead.  */);
+  Vmaximum_scroll_margin = make_float (0.25);
+
   DEFVAR_LISP ("display-pixels-per-inch",  Vdisplay_pixels_per_inch,
     doc: /* Pixels per inch value for non-window system displays.
 Value is a number or a cons (WIDTH-DPI . HEIGHT-DPI).  */);