Support precision mouse scrolling on MS-Windows

* src/w32fns.c (w32_wnd_proc): Pass the WM_SETTINGCHANGE message
to the Lisp thread.
* src/w32term.c (w32_construct_mouse_wheel): Support mice with
precision scrolling wheel.
(w32_get_mouse_wheel_vertical_delta): New function.
(w32_read_socket): When the WM_SETTINGCHANGE is received, call
'w32_get_mouse_wheel_vertical_delta'.
(w32_initialize): Call 'w32_get_mouse_wheel_vertical_delta' at
startup.
* src/nsterm.m (syms_of_nsterm):
* src/haikuterm.c (syms_of_haikuterm):
* src/xterm.c (syms_of_xterm): Remove window-system specific
variables for coalescing mwheel events.
* src/keyboard.c (syms_of_keyboard)
<mwheel-coalesce-scroll-events>: New variable, to replace the
above platform-specific ones.

* doc/lispref/commands.texi (Misc Events): Improve wording of the
description of mouse-wheel events.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/keyboard.c b/src/keyboard.c
index c98175aea0d..b3e6e5029be 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -12571,6 +12571,12 @@ other similar functions ignore input events in `while-no-input-ignore-events'.
 This flag may eventually be removed once this behavior is deemed safe.  */);
   input_pending_p_filter_events = true;
 
+  DEFVAR_BOOL ("mwheel-coalesce-scroll-events", mwheel_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.  */);
+  mwheel_coalesce_scroll_events = true;
+
   pdumper_do_now_and_after_load (syms_of_keyboard_for_pdumper);
 }