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.

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

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

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'.

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
 

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.

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.

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.

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.  */

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);

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;