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.

10 files changed, 68 insertions, 1 deletions

diff --git a/doc/emacs/ChangeLog b/doc/emacs/ChangeLog
index cd46d9fc474..d2f7a345566 100644
--- a/doc/emacs/ChangeLog
+++ b/doc/emacs/ChangeLog
@@ -1,3 +1,8 @@
+2014-12-14  Alan Mackenzie  <acm@muc.de>
+
+        * display.texi (Scrolling): fast-but-imprecise-scrolling.
+        Describe new variable.
+
 2014-12-14  Cameron Desautels  <camdez@gmail.com>
 
         * custom.texi (Saving Customizations): Mention
diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index a5555d58054..12a8e1b3eee 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -127,6 +127,19 @@ the mouse wheel (@pxref{Mouse Commands}); in general, it affects any
 command that has a non-@code{nil} @code{scroll-command} property.
 @xref{Property Lists,,, elisp, The Emacs Lisp Reference Manual}.
 
+@vindex fast-but-imprecise-scrolling
+  Sometimes, particularly when you hold down keys such as @kbd{C-v}
+and @kbd{M-v}, activating keyboard auto-repeat, Emacs fails to keep up
+with the rapid rate of scrolling requested; the display doesn't update
+and Emacs can become unresponsive to input for quite a long time.  You
+can counter this sluggishness by setting the variable
+@code{fast-but-imprecise-scrolling} to a non-@code{nil} value.  This
+instructs the scrolling commands not to fontify (@pxref{Font Lock})
+any unfontified text they scroll over, instead to assume it has the
+default face.  This can cause Emacs to scroll to somewhat wrong buffer
+positions when the faces in use are not all the same size, even with
+single (i.e. without auto-repeat) scrolling operations.
+
 @vindex scroll-up
 @vindex scroll-down
 @findex scroll-up-line
diff --git a/etc/ChangeLog b/etc/ChangeLog
index f344e2437b0..0a06cbd5c70 100644
--- a/etc/ChangeLog
+++ b/etc/ChangeLog
@@ -1,3 +1,7 @@
+2014-12-14  Alan Mackenzie  <acm@muc.de>
+
+        * NEWS: Add entry for fast-but-imprecise-scrolling.
+
 2014-12-14  Cameron Desautels  <camdez@gmail.com>
 
         * NEWS: Mention `custom-prompt-customize-unsaved-options'.
diff --git a/etc/NEWS b/etc/NEWS
index 37cb4b42e52..6b1a4e3627d 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -153,6 +153,12 @@ name (as returned from, for instance, `file-name-all-completions' is
 a directory file name.  It returns non-nil if the last character in
 the name is a forward slash.
 
++++
+** New variable `fast-but-imprecise-scrolling' inhibits
+fontification during full screen scrolling operations, giving less
+hesitant operation during auto-repeat of C-v, M-v at the cost of
+possible inaccuracies in the end position.
+
 
 * Editing Changes in Emacs 25.1
 
diff --git a/lisp/ChangeLog b/lisp/ChangeLog
index 288589d9e04..8a0d5181bae 100644
--- a/lisp/ChangeLog
+++ b/lisp/ChangeLog
@@ -1,3 +1,7 @@
+2014-12-14  Alan Mackenzie  <acm@muc.de>
+
+        * lisp/cus-start.el (all): Add fast-but-imprecise-scrolling.
+
 2014-12-14  Artur Malabarba  <bruce.connor.am@gmail.com>
 
         * let-alist.el: Add lexical binding.
diff --git a/lisp/cus-start.el b/lisp/cus-start.el
index 3b8885a8717..4049974841e 100644
--- a/lisp/cus-start.el
+++ b/lisp/cus-start.el
@@ -460,6 +460,7 @@ since it could result in memory overflow and make Emacs crash."
                               :value display-buffer)
                        (other :tag "Always (t)" :value t))
               "24.3")
+             (fast-but-imprecise-scrolling scrolling boolean "25.1")
              (window-resize-pixelwise windows boolean "24.4")
              ;; xdisp.c
              ;; The whitespace group is for whitespace.el.
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;