Expose pixel-wise wheel events to Lisp

* doc/lispref/commands.texi (Misc Events): Document changes to
wheel events.

* src/keyboard.c (make_lispy_event): Handle wheel events with
pixel delta data.
* src/termhooks.h (WHEEL_EVENT): Document changes to
WHEEL_EVENT args.
* src/xfns.c (syms_of_xfns): Declare new symbols.
* src/xterm.c (handle_one_xevent): Give wheel events pixel delta
data.
(x_coalesce_scroll_events): New user option.

doc/lispref/commands.texi

@@ -1994,7 +1994,18 @@ frame has already been made visible, Emacs has no work to do.
 These kinds of event are generated by moving a mouse wheel.  The
 @var{position} element is a mouse position list (@pxref{Click
 Events}), specifying the position of the mouse cursor when the event
-occurred.
+occurred.  The event may have additional arguments after
+@var{position}.  The third argument after @var{position}, if present,
+is a pair of the form @w{@code{(@var{x} . @var{y})}}, where @var{x}
+and @var{y} are the number of pixels to scroll by in each axis.
+
+@cindex pixel-resolution wheel events
+You can use @var{x} and @var{y} to determine how much the mouse wheel
+has actually moved at pixel resolution.
+
+For example, the pixelwise deltas could be used to scroll the display
+at pixel resolution, exactly according to the user's turning the mouse
+wheel.
 
 @vindex mouse-wheel-up-event
 @vindex mouse-wheel-down-event

src/keyboard.c

@@ -5980,7 +5980,11 @@ make_lispy_event (struct input_event *event)
 				      ASIZE (wheel_syms));
 	}
 
-        if (NUMBERP (event->arg))
+	if (CONSP (event->arg))
+	  return list5 (head, position, make_fixnum (double_click_count),
+			XCAR (event->arg), Fcons (XCAR (XCDR (event->arg)),
+						  XCAR (XCDR (XCDR (event->arg)))));
+        else if (NUMBERP (event->arg))
           return list4 (head, position, make_fixnum (double_click_count),
                         event->arg);
 	else if (event->modifiers & (double_modifier | triple_modifier))

src/termhooks.h

@@ -119,7 +119,10 @@ enum event_kind
 				   .timestamp gives a timestamp (in
 				   milliseconds) for the event.
                                    .arg may contain the number of
-                                   lines to scroll.  */
+                                   lines to scroll, or a list of
+				   the form (NUMBER-OF-LINES . (X Y)) where
+				   X and Y are the number of pixels
+				   on each axis to scroll by.  */
   HORIZ_WHEEL_EVENT,            /* A wheel event generated by a second
                                    horizontal wheel that is present on some
                                    mice. See WHEEL_EVENT.  */

src/xfns.c

@@ -8085,6 +8085,7 @@ eliminated in future versions of Emacs.  */);
 
 #ifdef HAVE_XINPUT2
   DEFSYM (Qxinput2, "xinput2");
+
   Fprovide (Qxinput2, Qnil);
 #endif
 

src/xterm.c

@@ -556,6 +556,7 @@ xi_reset_scroll_valuators_for_device_id (struct x_display_info *dpyinfo, int id)
     {
       valuator = &device->valuators[i];
       valuator->invalid_p = true;
+      valuator->emacs_value = 0.0;
     }
 
   return;
@@ -9921,8 +9922,6 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 #endif
 	    goto XI_OTHER;
 	  case XI_Motion:
-	    /* First test if there is some kind of scroll event
-	       here! */
 	    states = &xev->valuators;
 	    values = states->values;
 
@@ -9932,10 +9931,8 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 	      {
 		if (XIMaskIsSet (states->mask, i))
 		  {
-		    block_input ();
-
 		    struct xi_scroll_valuator_t *val;
-		    double delta;
+		    double delta, scroll_unit;
 
 		    delta = x_get_scroll_valuator_delta (dpyinfo, xev->deviceid,
 							 i, *values, &val);
@@ -9943,6 +9940,7 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 		    if (delta != DBL_MAX)
 		      {
 			f = mouse_or_wdesc_frame (dpyinfo, xev->event);
+			scroll_unit = pow (FRAME_PIXEL_HEIGHT (f), 2.0 / 3.0);
 			found_valuator = true;
 
 			if (signbit (delta) != signbit (val->emacs_value))
@@ -9950,15 +9948,16 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 
 			val->emacs_value += delta;
 
+			if (x_coalesce_scroll_events
+			    && (fabs (val->emacs_value) < 1))
+			  continue;
+
 			if (!f)
 			  {
 			    f = x_any_window_to_frame (dpyinfo, xev->event);
 
 			    if (!f)
-			      {
-				unblock_input ();
-				goto XI_OTHER;
-			      }
+			      goto XI_OTHER;
 			  }
 
 			bool s = signbit (val->emacs_value);
@@ -9975,13 +9974,26 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 			inev.ie.modifiers
 			  |= x_x_to_emacs_modifiers (dpyinfo,
 						     xev->mods.effective);
-			inev.ie.arg = Qnil;
+
+			if (val->horizontal)
+			  {
+			    inev.ie.arg
+			      = list3 (Qnil,
+				       make_float (val->emacs_value
+						   * scroll_unit),
+				       make_float (0));
+			  }
+                        else
+			  {
+			    inev.ie.arg = list3 (Qnil, make_float (0),
+						 make_float (val->emacs_value
+							     * scroll_unit));
+			  }
 
 			kbd_buffer_store_event_hold (&inev.ie, hold_quit);
 
 			val->emacs_value = 0;
 		      }
-		    unblock_input ();
 		    values++;
 		  }
 
@@ -15048,4 +15060,11 @@ gtk_window_move to set or store frame positions and disables some time
 consuming frame position adjustments.  In newer versions of GTK, Emacs
 always uses gtk_window_move and ignores the value of this variable.  */);
   x_gtk_use_window_move = true;
+
+  DEFVAR_BOOL ("x-coalesce-scroll-events", x_coalesce_scroll_events,
+	       doc: /* Non-nil means send a wheel event only for scrolling at least one screen line.
+Otherwise, a wheel event will be sent every time the mouse wheel is
+moved.  This option is only effective when Emacs is built with XInput
+2.  */);
+  x_coalesce_scroll_events = true;
 }