New feature optionally to accelerate auto-repeated scrolling.

src/xdisp.c: Remove "static" from declaration of
Qfontification_functions.

src/window.c (window_scroll): bind fontification-functions to nil when
scrolling by whole screens and fast-but-imprecise-scrolling is non-nil.
(syms_of_window): New DEFVAR_BOOL fast-but-imprecise-scrolling.

src/lisp.h (bool): Declare Qfontification_functions extern.

lisp/cus-start.el (all): Add fast-but-imprecise-scrolling.

doc/emacs/display.texi (Scrolling): fast-but-imprecise-scrolling.
Describe new variable.

etc/NEWS: Add entry for fast-but-imprecise-scrolling.

4 files changed, 35 insertions, 1 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index fd56186105b..46bf2801849 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,18 @@
+2014-12-14  Alan Mackenzie  <acm@muc.de>
+
+        New feature optionally to accelerate auto-repeated scrolling.
+
+        * xdisp.c: Remove "static" from declaration of
+        Qfontification_functions.
+
+        * window.c (window_scroll): bind fontification-functions to nil
+        when scrolling by whole screens and
+        fast-but-imprecise-scrolling is non-nil.
+        (syms_of_window): New DEFVAR_BOOL
+        fast-but-imprecise-scrolling.
+
+        * lisp.h (bool): Declare Qfontification_functions extern.
+
 2014-12-14  Eli Zaretskii  <eliz@gnu.org>
 
         Load system's default trusted Certificate Authorities if available.
diff --git a/src/lisp.h b/src/lisp.h
index a56c4a73bf8..37172c6b049 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -830,6 +830,7 @@ extern Lisp_Object Qwindowp;
 
 /* Defined in xdisp.c.  */
 extern Lisp_Object Qimage;
+extern Lisp_Object Qfontification_functions;
 
 
 /* Extract a value or address from a Lisp_Object.  */
diff --git a/src/window.c b/src/window.c
index 7c2b3ca29d2..0b766649b17 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4869,11 +4869,16 @@ window_internal_height (struct window *w)
 static void
 window_scroll (Lisp_Object window, EMACS_INT n, bool whole, int noerror)
 {
+  ptrdiff_t count = SPECPDL_INDEX ();
+
   immediate_quit = 1;
   n = clip_to_bounds (INT_MIN, n, INT_MAX);
 
   wset_redisplay (XWINDOW (window));
 
+  if (whole && Vfast_but_imprecise_scrolling)
+    specbind (Qfontification_functions, Qnil);
+
   /* If we must, use the pixel-based version which is much slower than
      the line-based one but can handle varying line heights.  */
   if (FRAME_WINDOW_P (XFRAME (XWINDOW (window)->frame)))
@@ -4881,6 +4886,8 @@ window_scroll (Lisp_Object window, EMACS_INT n, bool whole, int noerror)
   else
     window_scroll_line_based (window, n, whole, noerror);
 
+  unbind_to (count, Qnil);
+
   /* Bug#15957.  */
   XWINDOW (window)->window_end_valid = 0;
   immediate_quit = 0;
@@ -7478,6 +7485,17 @@ frame's character size, at least one window may get resized
 pixelwise even if this option is nil.  */);
   window_resize_pixelwise = 0;
 
+  DEFVAR_BOOL ("fast-but-imprecise-scrolling",
+               Vfast_but_imprecise_scrolling,
+               doc: /* When non-nil, accelerate scrolling operations.
+This comes into play when scrolling rapidly over previously
+unfontified buffer regions.  Only those portions of the buffer which
+are actually going to be displayed get fontified.
+
+Note that this optimization can cause the portion of the buffer
+displayed after a scrolling operation to be somewhat inaccurate.  */);
+  Vfast_but_imprecise_scrolling = 0;
+
   defsubr (&Sselected_window);
   defsubr (&Sminibuffer_window);
   defsubr (&Swindow_minibuffer_p);
diff --git a/src/xdisp.c b/src/xdisp.c
index f022790023b..48a2979dcde 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -347,7 +347,7 @@ Lisp_Object Qtext;
 /* Holds the list (error).  */
 static Lisp_Object list_of_error;
 
-static Lisp_Object Qfontification_functions;
+Lisp_Object Qfontification_functions;
 
 static Lisp_Object Qwrap_prefix;
 static Lisp_Object Qline_prefix;