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, 23 insertions, 4 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index c6e990d9082..15c700892bc 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -285,13 +285,17 @@ multiple variables, the order of priority is:
 @code{scroll-up-aggressively} / @code{scroll-down-aggressively}.
 
 @vindex scroll-margin
+@vindex maximum-scroll-margin
   The variable @code{scroll-margin} restricts how close point can come
 to the top or bottom of a window (even if aggressive scrolling
 specifies a fraction @var{f} that is larger than the window portion
-between the top and the bottom margins).  Its value is a number of screen
-lines; if point comes within that many lines of the top or bottom of
-the window, Emacs performs automatic scrolling.  By default,
-@code{scroll-margin} is 0.
+between the top and the bottom margins).  Its value is a number of
+screen lines; if point comes within that many lines of the top or
+bottom of the window, Emacs performs automatic scrolling.  By default,
+@code{scroll-margin} is 0.  The effective margin size is limited to a
+quarter of the window height by default, but this limit can be
+increased up to half (or decreased down to zero) by customizing
+@code{maximum-scroll-margin}.
 
 @node Horizontal Scrolling
 @section Horizontal Scrolling
diff --git a/doc/lispref/windows.texi b/doc/lispref/windows.texi
index 6f3de0c8a0e..affa28c9202 100644
--- a/doc/lispref/windows.texi
+++ b/doc/lispref/windows.texi
@@ -3924,6 +3924,21 @@ redisplay scrolls the text automatically (if possible) to move point
 out of the margin, closer to the center of the window.
 @end defopt
 
+@defopt maximum-scroll-margin
+This variable limits the effective value of @code{scroll-margin} to a
+fraction of the current window line height.  For example, if the
+current window has 20 lines and @code{maximum-scroll-margin} is 0.1,
+then the scroll margins will never be larger than 2 lines, no matter
+how big @code{scroll-margin} is.
+
+@code{maximum-scroll-margin} itself has a maximum value of 0.5, which
+allows setting margins large to keep the cursor at the middle line of
+the window (or two middle lines if the window has an even number of
+lines).  If it's set to a larger value (or any value other than a
+float between 0.0 and 0.5) then the default value of 0.25 will be used
+instead.
+@end defopt
+
 @defopt scroll-conservatively
 This variable controls how scrolling is done automatically when point
 moves off the screen (or into the scroll margin).  If the value is a