>From 87ad96cc13ca45eb5c78e3773cfb1458001f9d83 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Sun, 11 Sep 2016 11:09:57 -0400
Subject: [PATCH v2 3/3] Make limit on scroll-margin variable

TODO: `scroll-down-command' doesn't stay in the middle of the window.

* src/xdisp.c (maximum-scroll-margin): New variable.
* etc/NEWS: Mention it.
* src/window.c (window_scroll_pixel_based): Use it instead of hardcoding
division by 4 (Bug #5718).
* test/src/window-tests.el (window-tests-scroll-margin-whole-window):
(window-tests--point-in-middle-of-window-p): New test and helper
function.
---
 etc/NEWS                 |  4 ++++
 src/window.c             | 19 ++++++++++++++-----
 src/xdisp.c              |  8 ++++++++
 test/src/window-tests.el | 27 +++++++++++++++++++++++++++
 4 files changed, 53 insertions(+), 5 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index d91204b..f4fa98a 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -298,6 +298,10 @@ local part of a remote file name.  Thus, if you have a directory named
 "/~" on the remote host "foo", you can prevent it from being
 substituted by a home directory by writing it as "/foo:/:/~/file".
 
+** The new variable 'maximum-scroll-margin' allows having effective
+settings of 'scroll-margin' up to half the window size, instead of
+always restricting the margin to a quarter of the window.
+
 
 * Editing Changes in Emacs 26.1
 
diff --git a/src/window.c b/src/window.c
index b710333..f664597 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4802,11 +4802,20 @@ 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 0fd2a7c..55bb34a 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -31520,6 +31520,14 @@ syms_of_xdisp (void)
 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).  */);
diff --git a/test/src/window-tests.el b/test/src/window-tests.el
index f5cca73..1dcebef 100644
--- a/test/src/window-tests.el
+++ b/test/src/window-tests.el
@@ -81,4 +81,31 @@ window-tests-with-buffer-window
      (window-tests-scrolling (+ max-margin 1) max-margin)
      (window-tests-scrolling (+ max-margin 2) max-margin))))
 
+(defun window-tests--point-in-middle-of-window-p ()
+  (= (count-lines (window-start) (window-point))
+     (/ (1- (window-text-height)) 2)))
+
+(ert-deftest window-tests-scroll-margin-whole-window ()
+  "Test `maximum-scroll-margin' at 0.5.
+With a high `scroll-margin', this should keep cursor in the
+middle of the window."
+  (skip-unless (not noninteractive))
+  (let ((maximum-scroll-margin 0.5)
+        (scroll-margin 100))
+    (window-tests-with-buffer-window
+     (set-window-text-height nil 7)
+     (erase-buffer)
+     (insert (mapconcat #'number-to-string
+                        (number-sequence 1 200) "\n"))
+     (goto-char 1)
+     (sit-for 0)
+     (call-interactively 'scroll-up-command)
+     (sit-for 0)
+     (should (window-tests--point-in-middle-of-window-p))
+     (call-interactively 'scroll-up-command)
+     (sit-for 0)
+     (should (window-tests--point-in-middle-of-window-p))
+     (call-interactively 'scroll-down-command)
+     (sit-for 0)
+     (should (window-tests--point-in-middle-of-window-p)))))
 ;;; window-tests.el ends here
-- 
2.9.3