Re: Unfreezing the display during auto-repeated scrolling. [ Was: Aborting display. Is this possible? ]

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

Thanks for such a quick reply.

On Sun, Oct 26, 2014 at 06:45:47PM +0200, Eli Zaretskii wrote:
> > Date: Sun, 26 Oct 2014 12:43:33 +0000
> > From: Alan Mackenzie <acm@muc.de>
> > Cc: monnier@IRO.UMontreal.CA, emacs-devel@gnu.org

> > Here's what I've done:  Conceptually, ITs (i.e. "struct it"s, the
> > iterators in xdisp.c) are used for two different purposes:
> > (i) display;
> > (ii) calculation of displayed character sizes, positions etc.
> > I've distinguished these two cases by adding an extra boolean flag to IT,
> > called `not_for_display'.  The functions `init_iterator' and
> > `start_display' have got this as an extra boolean parameter.

> > Some of the ITs in window.c and xdisp.c are now designated
> > `not_for_display'.

> > `handle_fontified_prop' now checks this flag in IT.  If both the flag and
> > `use-default-face-for-fast-scrolling' are set, then fontification is not
> > done.  The existing face (usually default) is simply left there.  The
> > code doesn't also check for the event queue being non-empty - in
> > practice, this didn't work very well.

> > Here's the code.  Comments and criticism would be welcome.

> I'm afraid this is a non-starter.  Even if this feature is accepted
> (and I don't like it), it cannot be controlled on such a low level.
> You cannot possibly know, on this low level, which code will use the
> "not for display" functions, even if today they are only used in a
> context where ignoring font-lock faces might be OK.

The contract of xdisp.c says explicitly that the only visible display
function is redisplay, and this has no IT parameters.  So any IT with
not_for_display set should be "safe" if it is defined outside of xdisp.c

> You already marked as "not for display" line-pixel-height, which must
> be accurate in the results it returns, since otherwise it will violate
> its contract.  Likewise in resize_mini_window (which is definitely "for
> display"), redisplay_internal (likewise), etc.  OTOH, displaying
> strings (e.g., as part of mode-line display) will never bump into
> font-lock faces, so setting this flag there is not useful.

Sorry, I'd left unnecessary not_for_display's set.  I've cleared out
xdisp.c, such that the only function which sets not_for_display is
pos_visible_p.  The rest are not needed.

The other functions which set not_for_display are
(i) window_scroll_pixel_based (window.c);
(ii) Fvertical_motion (indent.c).

> So if we go this way, we are digging a hole for ourselves -- it is an
> impossible maintenance burden to have functions that don't exactly do
> what they say they do under some circumstances.

Yes.  But the current implementation doesn't work very well.  I'm trying
to fix it, because it's irritating.  I think I've made good progress.  I
can't see any other approach to fixing it at the moment.

> You should only make changes in window_scroll_* functions, ...

I honestly can't see how I could achieve what's wanted if I restrict
myself to window.c.

> ... and do that only under control of the
> use-default-face-for-fast-scrolling variable, so that the Lisp
> application that binds this variable to a non-nil value is the sole
> source of this behavior, and holds all the responsibility for binding
> it where it wants.  No code should disregard font-lock faces, except
> when this variable is non-nil.

The new feature I've added remains inactive when
use-default-face-for-fast-scrolling is nil.  The single effect when it is
set is to inhibit fontification when an IT.not_for_display flag is also
set.

I had in mind that use-default-face-for-fast-scrolling should be a user
option rather than something to be set by a major mode.  Something that
could upset exact scrolling should be turned on by a user, rather thaan
by a major mode.

> And I still don't understand why writing a simple "fast-scroll"
> function in Lisp, which just moves point N lines forward or backward,
> is not a better solution.  It's certainly easier and has exactly zero
> impact on our infrastructure.

That's precisely how Fscroll_up and Fscroll_down work.  I'd merely be
reinventing the wheel, and I'd face all the difficulties and special
cases which other hackers have long ago overcome.

To reiterate what I want: it is for an auto-repeated PageUp or PageDown
not to freeze the screen, either whilst depressed or after being
released.  I don't see how writing a new scroll command for these keys
would help.

Anyhow, I've now got the feature working for GUI windows.  It wasn't
difficult.

Here's the updated patch:



=== modified file 'src/dispextern.h'
--- src/dispextern.h	2014-10-15 10:22:15 +0000
+++ src/dispextern.h	2014-10-26 10:32:37 +0000
@@ -2275,6 +2275,12 @@
   /* True means cursor shouldn't be displayed here.  */
   bool_bf avoid_cursor_p : 1;
 
+  /* True means it is known that IT is only for calculating screen
+     occupancy, not for actually drawing elements on a screen.  This
+     suppresses fontification when user option
+     `use-default-face-for-fast-scrolling' is set. */
+  bool_bf not_for_display : 1;
+
   /* Display table in effect or null for none.  */
   struct Lisp_Char_Table *dp;
 
@@ -3204,10 +3210,10 @@
 void mark_window_display_accurate (Lisp_Object, int);
 void redisplay_preserve_echo_area (int);
 void init_iterator (struct it *, struct window *, ptrdiff_t,
-                    ptrdiff_t, struct glyph_row *, enum face_id);
+                    ptrdiff_t, struct glyph_row *, enum face_id, bool);
 void init_iterator_to_row_start (struct it *, struct window *,
                                  struct glyph_row *);
-void start_display (struct it *, struct window *, struct text_pos);
+void start_display (struct it *, struct window *, struct text_pos, bool);
 void move_it_vertically (struct it *, int);
 void move_it_vertically_backward (struct it *, int);
 void move_it_by_lines (struct it *, ptrdiff_t);

=== modified file 'src/dispnew.c'
--- src/dispnew.c	2014-10-14 12:45:41 +0000
+++ src/dispnew.c	2014-10-22 11:15:50 +0000
@@ -5131,7 +5131,7 @@
   Fset_buffer (w->contents);
   itdata = bidi_shelve_cache ();
   CLIP_TEXT_POS_FROM_MARKER (startp, w->start);
-  start_display (&it, w, startp);
+  start_display (&it, w, startp, 0);
   x0 = *x;
 
   /* First, move to the beginning of the row corresponding to *Y.  We

=== modified file 'src/indent.c'
--- src/indent.c	2014-08-28 01:59:29 +0000
+++ src/indent.c	2014-10-22 11:17:10 +0000
@@ -2009,7 +2009,7 @@
 
       itdata = bidi_shelve_cache ();
       SET_TEXT_POS (pt, PT, PT_BYTE);
-      start_display (&it, w, pt);
+      start_display (&it, w, pt, 1);
       first_x = it.first_visible_x;
       it_start = IT_CHARPOS (it);
 

=== modified file 'src/window.c'
--- src/window.c	2014-10-04 08:20:24 +0000
+++ src/window.c	2014-10-26 15:15:07 +0000
@@ -1799,7 +1799,7 @@
       CLIP_TEXT_POS_FROM_MARKER (startp, w->start);
 
       itdata = bidi_shelve_cache ();
-      start_display (&it, w, startp);
+      start_display (&it, w, startp, 0);
       move_it_vertically (&it, window_box_height (w));
       if (it.current_y < it.last_visible_y)
 	move_it_past_eol (&it);
@@ -4923,7 +4923,7 @@
       /* Move backward half the height of the window.  Performance note:
 	 vmotion used here is about 10% faster, but would give wrong
 	 results for variable height lines.  */
-      init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID);
+      init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID, 1);
       it.current_y = it.last_visible_y;
       move_it_vertically_backward (&it, window_box_height (w) / 2);
 
@@ -4934,7 +4934,7 @@
 	 start of the line containing PT in this case.  */
       if (it.current_y <= 0)
 	{
-	  init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID);
+        init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID, 1);
 	  move_it_vertically_backward (&it, 0);
 	  it.current_y = 0;
 	}
@@ -5015,7 +5015,7 @@
 	  || !SYMBOLP (KVAR (current_kboard, Vlast_command))
 	  || NILP (Fget (KVAR (current_kboard, Vlast_command), Qscroll_command)))
 	{
-	  start_display (&it, w, start);
+          start_display (&it, w, start, 1);
 	  move_it_to (&it, PT, -1, -1, -1, MOVE_TO_POS);
 	  window_scroll_pixel_based_preserve_y = it.current_y;
 	  window_scroll_pixel_based_preserve_x = it.current_x;
@@ -5027,7 +5027,7 @@
 
   /* Move iterator it from start the specified distance forward or
      backward.  The result is the new window start.  */
-  start_display (&it, w, start);
+  start_display (&it, w, start, 1);
   if (whole)
     {
       ptrdiff_t start_pos = IT_CHARPOS (it);
@@ -5252,7 +5252,7 @@
       else if (window_scroll_pixel_based_preserve_y >= 0)
 	{
 	  SET_TEXT_POS_FROM_MARKER (start, w->start);
-	  start_display (&it, w, start);
+	  start_display (&it, w, start, 1);
 	  /* It would be wrong to subtract CURRENT_HEADER_LINE_HEIGHT
 	     here because we called start_display again and did not
 	     alter it.current_y this time.  */
@@ -5310,6 +5310,20 @@
   bool adjust_old_pointm = !NILP (Fequal (Fwindow_point (window),
 					  Fwindow_old_point (window)));
 
+#if 0
+  Lisp_Object Fhighest_fontified = intern ("highest-fontified");
+  Lisp_Object Flowest_fontified = intern ("lowest-fontified");
+  Lisp_Object Fdump_fontification_diags = intern ("dump-fontification-diags");
+  Lisp_Object pre_point = Fpoint (), post_point, pre_low_f, post_low_f, pre_high_f, post_high_f;
+
+  Lisp_Object dfd_array [7];
+  struct gcpro gcpro1, gcpro2, gcpro3, gcpro4, gcpro5, gcpro6;
+  GCPRO6 (Fhighest_fontified, Fdump_fontification_diags,
+          pre_point, post_point, pre_high_f, post_high_f);
+  pre_low_f = Ffuncall (1, (Lisp_Object []){Flowest_fontified});
+  pre_high_f = Ffuncall (0, (Lisp_Object []){Fhighest_fontified});
+#endif
+
   /* If scrolling screen-fulls, compute the number of lines to
      scroll from the window's height.  */
   if (whole)
@@ -5355,7 +5369,12 @@
   if (lose)
     {
       if (noerror)
-	return;
+          {
+#if 0
+          UNGCPRO;
+#endif
+          return;
+          }
       else
 	xsignal0 (Qbeginning_of_buffer);
     }
@@ -5436,7 +5455,12 @@
   else
     {
       if (noerror)
-	return;
+          {
+#if 0
+          UNGCPRO;
+#endif
+          return;
+          }
       else
 	xsignal0 (Qend_of_buffer);
     }
@@ -5447,6 +5471,21 @@
 		  ? make_number (BUF_PT (XBUFFER (w->contents)))
 		  : Fmarker_position (w->pointm)),
 		 w->contents);
+
+#if 0
+  post_low_f = Ffuncall (1, (Lisp_Object []){Flowest_fontified});
+  post_high_f = Ffuncall (1, (Lisp_Object []){Fhighest_fontified});
+  post_point = Fpoint ();
+  dfd_array [0] = Fdump_fontification_diags;
+  dfd_array [1] = pre_low_f;
+  dfd_array [2] = pre_high_f;
+  dfd_array [3] = pre_point;
+  dfd_array [4] = post_low_f;
+  dfd_array [5] = post_high_f;
+  dfd_array [6] = post_point;
+  Ffuncall (7, dfd_array);
+  UNGCPRO;
+#endif
 }
 
 
@@ -5694,7 +5733,7 @@
   CLIP_TEXT_POS_FROM_MARKER (start, w->start);
 
   itdata = bidi_shelve_cache ();
-  start_display (&it, w, start);
+  start_display (&it, w, start, 0);
   move_it_vertically (&it, height);
   bottom_y = line_bottom_y (&it);
   bidi_unshelve_cache (itdata, 0);
@@ -5798,7 +5837,7 @@
 	  void *itdata = bidi_shelve_cache ();
 
 	  SET_TEXT_POS (pt, PT, PT_BYTE);
-	  start_display (&it, w, pt);
+	  start_display (&it, w, pt, 0);
 	  move_it_vertically_backward (&it, window_box_height (w) / 2);
 	  charpos = IT_CHARPOS (it);
 	  bytepos = IT_BYTEPOS (it);
@@ -5816,7 +5855,7 @@
 	  iarg = - max (-iarg, this_scroll_margin);
 
 	  SET_TEXT_POS (pt, PT, PT_BYTE);
-	  start_display (&it, w, pt);
+	  start_display (&it, w, pt, 0);
 
 	  /* Be sure we have the exact height of the full line containing PT.  */
 	  move_it_by_lines (&it, 0);
@@ -5855,7 +5894,7 @@
 	    }
 
 	  /* Now find the new top line (starting position) of the window.  */
-	  start_display (&it, w, pt);
+	  start_display (&it, w, pt, 0);
 	  it.current_y = 0;
 	  move_it_vertically_backward (&it, h);
 

=== modified file 'src/xdisp.c'
--- src/xdisp.c	2014-10-24 09:58:43 +0000
+++ src/xdisp.c	2014-10-26 18:48:23 +0000
@@ -1289,7 +1289,7 @@
       set_buffer_internal_1 (XBUFFER (w->contents));
     }
   SET_TEXT_POS (pt, PT, PT_BYTE);
-  start_display (&it, w, pt);
+  start_display (&it, w, pt, 0);
   it.vpos = it.current_y = 0;
   last_height = 0;
   result = make_number (line_bottom_y (&it));
@@ -1434,7 +1434,7 @@
       = display_mode_line (w, HEADER_LINE_FACE_ID,
 			   BVAR (current_buffer, header_line_format));
 
-  start_display (&it, w, top);
+  start_display (&it, w, top, 1);
   move_it_to (&it, charpos, -1, it.last_visible_y - 1, -1,
 	      (charpos >= 0 ? MOVE_TO_POS : 0) | MOVE_TO_Y);
 
@@ -1508,7 +1508,7 @@
 		     the previous buffer position is also displayed
 		     from a display vector, we need to consume all of
 		     the glyphs from that display vector.  */
-		  start_display (&it2, w, top);
+		  start_display (&it2, w, top, 1);
 		  move_it_to (&it2, charpos - 1, -1, -1, -1, MOVE_TO_POS);
 		  /* If we didn't get to CHARPOS - 1, there's some
 		     replacing display property at that position, and
@@ -1590,7 +1590,7 @@
 		  end = XFASTINT (endpos);
 		  /* Move to the last buffer position before the
 		     display property.  */
-		  start_display (&it3, w, top);
+		  start_display (&it3, w, top, 1);
 		  if (start > CHARPOS (top))
 		    move_it_to (&it3, start - 1, -1, -1, -1, MOVE_TO_POS);
 		  /* Move forward one more line if the position before
@@ -1625,7 +1625,7 @@
 			 display string could be _after_ the display
 			 property in the logical order.  Use the
 			 smallest vertical position of these two.  */
-		      start_display (&it3, w, top);
+                    start_display (&it3, w, top, 1);
 		      move_it_to (&it3, end + 1, -1, -1, -1, MOVE_TO_POS);
 		      if (it3.current_y < top_y)
 			top_y = it3.current_y;
@@ -1633,7 +1633,7 @@
 		  /* Move from the top of the window to the beginning
 		     of the display line where the display string
 		     begins.  */
-		  start_display (&it3, w, top);
+		  start_display (&it3, w, top, 1);
 		  move_it_to (&it3, -1, 0, top_y, -1, MOVE_TO_X | MOVE_TO_Y);
 		  /* If it3_moved stays zero after the 'while' loop
 		     below, that means we already were at a newline
@@ -2795,12 +2795,18 @@
    If ROW is null and BASE_FACE_ID is equal to MODE_LINE_FACE_ID,
    MODE_LINE_INACTIVE_FACE_ID, or HEADER_LINE_FACE_ID, the iterator
    will be initialized to use the corresponding mode line glyph row of
-   the desired matrix of W.  */
+   the desired matrix of W.
+
+   NOT_FOR_DISPLAY can be set to true if it is known the iterator will
+   not be used for display.  This optimization prevents this iterator
+   from performing fontification when the user option
+   USE-DEFAULT-FACE-FOR-FAST-SCROLLING is enabled. */
 
 void
 init_iterator (struct it *it, struct window *w,
 	       ptrdiff_t charpos, ptrdiff_t bytepos,
-	       struct glyph_row *row, enum face_id base_face_id)
+	       struct glyph_row *row, enum face_id base_face_id,
+               bool not_for_display)
 {
   enum face_id remapped_base_face_id = base_face_id;
 
@@ -2840,6 +2846,7 @@
   it->current.overlay_string_index = -1;
   it->current.dpvec_index = -1;
   it->base_face_id = remapped_base_face_id;
+  it->not_for_display = not_for_display;
   it->string = Qnil;
   IT_STRING_CHARPOS (*it) = IT_STRING_BYTEPOS (*it) = -1;
   it->paragraph_embedding = L2R;
@@ -3097,13 +3104,15 @@
 /* Initialize IT for the display of window W with window start POS.  */
 
 void
-start_display (struct it *it, struct window *w, struct text_pos pos)
+start_display (struct it *it, struct window *w, struct text_pos pos,
+               bool not_for_display)
 {
   struct glyph_row *row;
   int first_vpos = WINDOW_WANTS_HEADER_LINE_P (w) ? 1 : 0;
 
   row = w->desired_matrix->rows + first_vpos;
-  init_iterator (it, w, CHARPOS (pos), BYTEPOS (pos), row, DEFAULT_FACE_ID);
+  init_iterator (it, w, CHARPOS (pos), BYTEPOS (pos), row, DEFAULT_FACE_ID,
+                 not_for_display);
   it->first_vpos = first_vpos;
 
   /* Don't reseat to previous visible line start if current start
@@ -3243,7 +3252,7 @@
      newline before the row start, such a POS will not be in a string,
      but the call to init_iterator below will move us to the
      after-string.  */
-  init_iterator (it, w, charpos, bytepos, NULL, DEFAULT_FACE_ID);
+  init_iterator (it, w, charpos, bytepos, NULL, DEFAULT_FACE_ID, 0);
 
   /* This only scans the current chunk -- it should scan all chunks.
      However, OVERLAY_STRING_CHUNK_SIZE has been increased from 3 in 21.1
@@ -3867,7 +3876,10 @@
 	  prop = Fget_char_property (pos, Qfontified, Qnil),
 	  /* Ignore the special cased nil value always present at EOB since
 	     no amount of fontifying will be able to change it.  */
-	  NILP (prop) && IT_CHARPOS (*it) < Z))
+	  NILP (prop) && IT_CHARPOS (*it) < Z)
+      && (NILP (Vuse_default_face_for_fast_scrolling)
+          || !it->not_for_display
+          /* || NILP (Finput_pending_p (Qnil)) */))
     {
       ptrdiff_t count = SPECPDL_INDEX ();
       Lisp_Object val;
@@ -9866,7 +9878,7 @@
 
   itdata = bidi_shelve_cache ();
   SET_TEXT_POS (startp, start, CHAR_TO_BYTE (start));
-  start_display (&it, w, startp);
+  start_display (&it, w, startp, 0);
 
   if (NILP (x_limit))
     x = move_it_to (&it, end, -1, max_y, -1, MOVE_TO_POS | MOVE_TO_Y);
@@ -10915,7 +10927,7 @@
 	  set_buffer_internal (XBUFFER (w->contents));
 	}
 
-      init_iterator (&it, w, BEGV, BEGV_BYTE, NULL, DEFAULT_FACE_ID);
+      init_iterator (&it, w, BEGV, BEGV_BYTE, NULL, DEFAULT_FACE_ID, 0);
 
       /* Compute the max. number of lines specified by the user.  */
       if (FLOATP (Vmax_mini_window_height))
@@ -10946,7 +10958,7 @@
       if (height > max_height)
 	{
 	  height = (max_height / unit) * unit;
-	  init_iterator (&it, w, ZV, ZV_BYTE, NULL, DEFAULT_FACE_ID);
+	  init_iterator (&it, w, ZV, ZV_BYTE, NULL, DEFAULT_FACE_ID, 0);
 	  move_it_vertically_backward (&it, height - unit);
 	  start = it.current.pos;
 	}
@@ -11626,7 +11638,7 @@
       mode_line_target = MODE_LINE_TITLE;
       title_start = MODE_LINE_NOPROP_LEN (0);
       init_iterator (&it, XWINDOW (f->selected_window), -1, -1,
-		     NULL, DEFAULT_FACE_ID);
+		     NULL, DEFAULT_FACE_ID, 0);
       display_mode_element (&it, 0, -1, -1, fmt, Qnil, 0);
       len = MODE_LINE_NOPROP_LEN (title_start);
       title = mode_line_noprop_buf + title_start;
@@ -12355,7 +12367,7 @@
 
   /* Initialize an iterator for iteration over
      F->desired_tool_bar_string in the tool-bar window of frame F.  */
-  init_iterator (&it, w, -1, -1, temp_row, TOOL_BAR_FACE_ID);
+  init_iterator (&it, w, -1, -1, temp_row, TOOL_BAR_FACE_ID, 0);
   temp_row->reversed_p = false;
   it.first_visible_x = 0;
   it.last_visible_x = WINDOW_PIXEL_WIDTH (w);
@@ -12437,7 +12449,7 @@
     return 0;
 
   /* Set up an iterator for the tool-bar window.  */
-  init_iterator (&it, w, -1, -1, w->desired_matrix->rows, TOOL_BAR_FACE_ID);
+  init_iterator (&it, w, -1, -1, w->desired_matrix->rows, TOOL_BAR_FACE_ID, 0);
   it.first_visible_x = 0;
   it.last_visible_x = WINDOW_PIXEL_WIDTH (w);
   row = it.glyph_row;
@@ -13709,7 +13721,7 @@
 
 	  /* Note that start_display will handle the case that the
 	     line starting at tlbufpos is a continuation line.  */
-	  start_display (&it, w, tlbufpos);
+	  start_display (&it, w, tlbufpos, 0);
 
 	  /* Implementation note: It this still necessary?  */
 	  if (it.current_x != this_line_start_x)
@@ -13825,7 +13837,7 @@
 	     PT may be in invisible text.  If so, we will end at the
 	     next visible position.  */
 	  init_iterator (&it, w, CHARPOS (tlbufpos), BYTEPOS (tlbufpos),
-			 NULL, DEFAULT_FACE_ID);
+			 NULL, DEFAULT_FACE_ID, 0);
 	  it.current_x = this_line_start_x;
 	  it.current_y = this_line_y;
 	  it.vpos = this_line_vpos;
@@ -15138,7 +15150,7 @@
 
       /* Compute the pixel ypos of the scroll margin, then move IT to
 	 either that ypos or PT, whichever comes first.  */
-      start_display (&it, w, startp);
+      start_display (&it, w, startp, 0);
       scroll_margin_y = it.last_visible_y - this_scroll_margin
 	- frame_line_height * extra_scroll_margin_lines;
       move_it_to (&it, PT, -1, scroll_margin_y - 1, -1,
@@ -15208,7 +15220,7 @@
       if (amount_to_scroll <= 0)
 	return SCROLLING_FAILED;
 
-      start_display (&it, w, startp);
+      start_display (&it, w, startp, 0);
       if (arg_scroll_conservatively <= scroll_limit)
 	move_it_vertically (&it, amount_to_scroll);
       else
@@ -15250,7 +15262,7 @@
 	{
 	  int y_start;
 
-	  start_display (&it, w, startp);
+	  start_display (&it, w, startp, 0);
 	  y_start = it.current_y;
 	  move_it_vertically (&it, this_scroll_margin);
 	  scroll_margin_pos = it.current.pos;
@@ -15274,7 +15286,7 @@
 	     Give up if distance is greater than scroll_max or if we
 	     didn't reach the scroll margin position.  */
 	  SET_TEXT_POS (pos, PT, PT_BYTE);
-	  start_display (&it, w, pos);
+	  start_display (&it, w, pos, 0);
 	  y0 = it.current_y;
 	  y_to_move = max (it.last_visible_y,
 			   max (scroll_max, 10 * frame_line_height));
@@ -15290,7 +15302,7 @@
 	  dy += y_offset;
 
 	  /* Compute new window start.  */
-	  start_display (&it, w, startp);
+	  start_display (&it, w, startp, 0);
 
 	  if (arg_scroll_conservatively)
 	    amount_to_scroll = max (dy, frame_line_height *
@@ -15399,7 +15411,7 @@
 	 because find_newline is fast (newline cache).  */
       row = w->desired_matrix->rows + (WINDOW_WANTS_HEADER_LINE_P (w) ? 1 : 0);
       init_iterator (&it, w, CHARPOS (start_pos), BYTEPOS (start_pos),
-		     row, DEFAULT_FACE_ID);
+		     row, DEFAULT_FACE_ID, 0);
       reseat_at_previous_visible_line_start (&it);
 
       /* If the line start is "too far" away from the window start,
@@ -15871,7 +15883,7 @@
 	}
 
       SET_TEXT_POS_FROM_MARKER (startp, w->start);
-      start_display (&it, w, startp);
+      start_display (&it, w, startp, 0);
       it.last_visible_x = INT_MAX;
       whole = move_it_to (&it, -1, INT_MAX, window_box_height (w), -1,
 			  MOVE_TO_X | MOVE_TO_Y);
@@ -16184,7 +16196,7 @@
       ptrdiff_t it_charpos;
 
       w->optional_new_start = 0;
-      start_display (&it, w, startp);
+      start_display (&it, w, startp, 0);
       move_it_to (&it, PT, 0, it.last_visible_y, -1,
 		  MOVE_TO_POS | MOVE_TO_X | MOVE_TO_Y);
       /* Record IT's position now, since line_bottom_y might change
@@ -16577,7 +16589,7 @@
     w->base_line_number = 0;
 
   /* Determine the window start relative to point.  */
-  init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID);
+  init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID, 0);
   it.current_y = it.last_visible_y;
   if (centering_position < 0)
     {
@@ -16606,7 +16618,7 @@
 	  void *it1data = NULL;
 
 	  SAVE_IT (it1, it, it1data);
-	  start_display (&it1, w, startp);
+	  start_display (&it1, w, startp, 0);
 	  move_it_vertically (&it1, margin * frame_line_height);
 	  margin_pos = IT_CHARPOS (it1);
 	  RESTORE_IT (&it, &it, it1data);
@@ -16669,7 +16681,7 @@
      containing PT in this case.  */
   if (it.current_y <= 0)
     {
-      init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID);
+      init_iterator (&it, w, PT, PT_BYTE, NULL, DEFAULT_FACE_ID, 0);
       move_it_vertically_backward (&it, 0);
       it.current_y = 0;
     }
@@ -17013,7 +17025,7 @@
   overlay_arrow_seen = 0;
 
   /* Initialize iterator and info to start at POS.  */
-  start_display (&it, w, pos);
+  start_display (&it, w, pos, 0);
   it.glyph_row->reversed_p = false;
 
   /* Display all lines of W.  */
@@ -17161,7 +17173,7 @@
 	 last_text_row is set to the last row displayed that displays
 	 text.  Note that it.vpos == 0 if or if not there is a
          header-line; it's not the same as the MATRIX_ROW_VPOS!  */
-      start_display (&it, w, new_start);
+      start_display (&it, w, new_start, 0);
       w->cursor.vpos = -1;
       last_text_row = last_reused_text_row = NULL;
 
@@ -18142,7 +18154,7 @@
     {
       /* There are no reusable lines at the start of the window.
 	 Start displaying in the first text line.  */
-      start_display (&it, w, start);
+      start_display (&it, w, start, 0);
       it.vpos = it.first_vpos;
       start_pos = it.current.pos;
     }
@@ -18932,7 +18944,7 @@
   int n_glyphs_before;
 
   set_buffer_temp (buffer);
-  init_iterator (&it, w, -1, -1, &scratch_glyph_row, DEFAULT_FACE_ID);
+  init_iterator (&it, w, -1, -1, &scratch_glyph_row, DEFAULT_FACE_ID, 0);
   scratch_glyph_row.reversed_p = false;
   it.glyph_row->used[TEXT_AREA] = 0;
   SET_TEXT_POS (it.position, 0, 0);
@@ -21228,7 +21240,7 @@
 
       /* Setup the arena.  */
       SET_TEXT_POS (pt, PT, PT_BYTE);
-      start_display (&it, w, pt);
+      start_display (&it, w, pt, 0);
 
       if (it.cmp_it.id < 0
 	  && it.method == GET_FROM_STRING
@@ -21319,7 +21331,7 @@
 	{
 	  if (pt_x > 0)
 	    {
-	      start_display (&it, w, pt);
+              start_display (&it, w, pt, 0);
 	      reseat_at_previous_visible_line_start (&it);
 	      it.current_x = it.current_y = it.hpos = 0;
 	      if (pt_vpos != 0)
@@ -21622,7 +21634,7 @@
 
 #if defined (USE_X_TOOLKIT) || defined (USE_GTK)
   eassert (!FRAME_WINDOW_P (f));
-  init_iterator (&it, w, -1, -1, f->desired_matrix->rows, MENU_FACE_ID);
+  init_iterator (&it, w, -1, -1, f->desired_matrix->rows, MENU_FACE_ID, 0);
   it.first_visible_x = 0;
   it.last_visible_x = FRAME_PIXEL_WIDTH (f);
 #elif defined (HAVE_X_WINDOWS) /* X without toolkit.  */
@@ -21633,7 +21645,7 @@
       struct window *menu_w;
       menu_w = XWINDOW (f->menu_bar_window);
       init_iterator (&it, menu_w, -1, -1, menu_w->desired_matrix->rows,
-		     MENU_FACE_ID);
+		     MENU_FACE_ID, 0);
       it.first_visible_x = 0;
       it.last_visible_x = FRAME_PIXEL_WIDTH (f);
     }
@@ -21643,7 +21655,7 @@
       /* This is a TTY frame, i.e. character hpos/vpos are used as
 	 pixel x/y.  */
       init_iterator (&it, w, -1, -1, f->desired_matrix->rows,
-		     MENU_FACE_ID);
+		     MENU_FACE_ID, 0);
       it.first_visible_x = 0;
       it.last_visible_x = FRAME_COLS (f);
     }
@@ -21759,7 +21771,7 @@
   if (y >= f->desired_matrix->nrows)
     return;
 
-  init_iterator (&it, w, -1, -1, f->desired_matrix->rows + y, MENU_FACE_ID);
+  init_iterator (&it, w, -1, -1, f->desired_matrix->rows + y, MENU_FACE_ID, 0);
   it.first_visible_x = 0;
   it.last_visible_x = FRAME_COLS (f) - 1;
   row = it.glyph_row;
@@ -21932,7 +21944,7 @@
   struct face *face;
   ptrdiff_t count = SPECPDL_INDEX ();
 
-  init_iterator (&it, w, -1, -1, NULL, face_id);
+  init_iterator (&it, w, -1, -1, NULL, face_id, 0);
   /* Don't extend on a previously drawn mode-line.
      This may happen if called from pos_visible_p.  */
   it.glyph_row->enabled_p = false;
@@ -22651,7 +22663,7 @@
   Fselect_window (window, Qt);
   set_buffer_internal_1 (XBUFFER (buffer));
 
-  init_iterator (&it, w, -1, -1, NULL, face_id);
+  init_iterator (&it, w, -1, -1, NULL, face_id, 0);
 
   if (no_props)
     {
@@ -31056,6 +31068,18 @@
 	       doc: /*  */);
   Vredisplay__mode_lines_cause
     = Fmake_vector (make_number (100), make_number (0));
+
+  DEFVAR_LISP ("use-default-face-for-fast-scrolling",
+               Vuse_default_face_for_fast_scrolling,
+               doc: /* When non-nil, accelerate repeated 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 to
+be slightly different.  */);
+
+  Vuse_default_face_for_fast_scrolling = Qnil;
 }
 
 



> Thanks.

-- 
Alan Mackenzie (Nuremberg, Germany).