patch 8.2.4902: mouse wheel scrolling is inconsistent

Problem:    Mouse wheel scrolling is inconsistent.
Solution:   Use the MS-Winows system setting. (closes #10368)

runtime/doc/scroll.txt

@@ -237,31 +237,34 @@ it works depends on your system.  It might also work in an xterm
 |xterm-mouse-wheel|.  By default only vertical scroll wheels are supported,
 but some GUIs also support horizontal scroll wheels.
 
-For the Win32 GUI the scroll action is hard coded.  It works just like
-dragging the scrollbar of the current window.  How many lines are scrolled
-depends on your mouse driver.  If the scroll action causes input focus
-problems, see |intellimouse-wheel-problems|.
+On MS-Windows, if the scroll action causes input focus -problems, see
+|intellimouse-wheel-problems|.
 
-For the X11 GUIs (Motif and GTK) scrolling the wheel generates key
+For Win32 and the X11 GUIs (Motif and GTK) scrolling the wheel generates key
 presses <ScrollWheelUp>, <ScrollWheelDown>, <ScrollWheelLeft> and
 <ScrollWheelRight>.  For example, if you push the scroll wheel upwards a
 <ScrollWheelUp> key press is generated causing the window to scroll upwards
 (while the text is actually moving downwards).  The default action for these
 keys are:
-    <ScrollWheelUp>	    scroll three lines up	*<ScrollWheelUp>*
+    <ScrollWheelUp>	    scroll N lines up	        *<ScrollWheelUp>*
     <S-ScrollWheelUp>	    scroll one page up		*<S-ScrollWheelUp>*
     <C-ScrollWheelUp>	    scroll one page up		*<C-ScrollWheelUp>*
-    <ScrollWheelDown>	    scroll three lines down	*<ScrollWheelDown>*
+    <ScrollWheelDown>	    scroll N lines down	        *<ScrollWheelDown>*
     <S-ScrollWheelDown>	    scroll one page down	*<S-ScrollWheelDown>*
     <C-ScrollWheelDown>	    scroll one page down	*<C-ScrollWheelDown>*
-    <ScrollWheelLeft>	    scroll six columns left	*<ScrollWheelLeft>*
+    <ScrollWheelLeft>	    scroll N columns left	*<ScrollWheelLeft>*
     <S-ScrollWheelLeft>	    scroll one page left	*<S-ScrollWheelLeft>*
     <C-ScrollWheelLeft>	    scroll one page left	*<C-ScrollWheelLeft>*
-    <ScrollWheelRight>	    scroll six columns right	*<ScrollWheelRight>*
+    <ScrollWheelRight>	    scroll N columns right	*<ScrollWheelRight>*
     <S-ScrollWheelRight>    scroll one page right	*<S-ScrollWheelRight>*
     <C-ScrollWheelRight>    scroll one page right	*<C-ScrollWheelRight>*
 This should work in all modes, except when editing the command line.
 
+The value of N depends on the system.  By default Vim scrolls three lines when
+moving vertically, and six columns when moving horizontally.  On MS-Windows
+the amount of lines and columns for each scroll action is taken from the
+system-wide settings.
+
 Note that horizontal scrolling only works if 'nowrap' is set.  Also, unless
 the "h" flag in 'guioptions' is set, the cursor moves to the longest visible
 line if the cursor line is about to be scrolled off the screen (similarly to

src/gui_w32.c

@@ -230,6 +230,10 @@ gui_mch_set_rendering_options(char_u *s)
 # define SPI_GETWHEELSCROLLCHARS	0x006C
 #endif
 
+#ifndef SPI_SETWHEELSCROLLCHARS
+# define SPI_SETWHEELSCROLLCHARS	0x006D
+#endif
+
 #ifdef PROTO
 /*
  * Define a few things for generating prototypes.  This is just to avoid
@@ -4117,19 +4121,33 @@ gui_mswin_get_menu_height(
 /*
  * Setup for the Intellimouse
  */
+    static long
+mouse_vertical_scroll_step(void)
+{
+    UINT val;
+    if (SystemParametersInfo(SPI_GETWHEELSCROLLLINES, 0, &val, 0))
+	return (val != WHEEL_PAGESCROLL) ? (long)val : -1;
+    return 3; // Safe default;
+}
+
+    static long
+mouse_horizontal_scroll_step(void)
+{
+    UINT val;
+    if (SystemParametersInfo(SPI_GETWHEELSCROLLCHARS, 0, &val, 0))
+	return (long)val;
+    return 3; // Safe default;
+}
+
     static void
 init_mouse_wheel(void)
 {
-    // Reasonable default values.
-    mouse_scroll_lines = 3;
-    mouse_scroll_chars = 3;
-
-    // if NT 4.0+ (or Win98) get scroll lines directly from system
-    SystemParametersInfo(SPI_GETWHEELSCROLLLINES, 0, &mouse_scroll_lines, 0);
-    SystemParametersInfo(SPI_GETWHEELSCROLLCHARS, 0, &mouse_scroll_chars, 0);
+    // Get the default values for the horizontal and vertical scroll steps from
+    // the system.
+    mouse_set_vert_scroll_step(mouse_vertical_scroll_step());
+    mouse_set_hor_scroll_step(mouse_horizontal_scroll_step());
 }
 
-
 /*
  * Intellimouse wheel handler.
  * Treat a mouse wheel event as if it were a scroll request.
@@ -4137,16 +4155,10 @@ init_mouse_wheel(void)
     static void
 _OnMouseWheel(HWND hwnd, short zDelta, LPARAM param, int horizontal)
 {
-    int		i;
-    int		amount;
     int		button;
     win_T	*wp;
     int		modifiers, kbd_modifiers;
 
-    // Initializes mouse_scroll_chars too.
-    if (mouse_scroll_lines == 0)
-	init_mouse_wheel();
-
     wp = gui_mouse_window(FIND_POPUP);
 
 #ifdef FEAT_PROP_POPUP
@@ -4185,23 +4197,9 @@ _OnMouseWheel(HWND hwnd, short zDelta, LPARAM param, int horizontal)
     // Translate the scroll event into an event that Vim can process so that
     // the user has a chance to map the scrollwheel buttons.
     if (horizontal)
-    {
 	button = zDelta >= 0 ? MOUSE_6 : MOUSE_7;
-	if (mouse_scroll_chars > 0
-			       && mouse_scroll_chars < MAX(wp->w_width - 2, 1))
-	    amount = mouse_scroll_chars;
-	else
-	    amount = MAX(wp->w_width - 2, 1);
-    }
     else
-    {
 	button = zDelta >= 0 ? MOUSE_4 : MOUSE_5;
-	if (mouse_scroll_lines > 0
-			      && mouse_scroll_lines < MAX(wp->w_height - 2, 1))
-	    amount = mouse_scroll_lines;
-	else
-	    amount = MAX(wp->w_height - 2, 1);
-    }
 
     kbd_modifiers = get_active_modifiers();
 
@@ -4213,8 +4211,7 @@ _OnMouseWheel(HWND hwnd, short zDelta, LPARAM param, int horizontal)
 	modifiers |= MOUSE_ALT;
 
     mch_disable_flush();
-    for (i = amount; i > 0; --i)
-	gui_send_mouse_event(button, GET_X_LPARAM(param), GET_Y_LPARAM(param),
+    gui_send_mouse_event(button, GET_X_LPARAM(param), GET_Y_LPARAM(param),
 		FALSE, kbd_modifiers);
     mch_enable_flush();
     gui_may_flush();
@@ -4296,12 +4293,10 @@ _OnSettingChange(UINT param)
     switch (param)
     {
 	case SPI_SETWHEELSCROLLLINES:
-	    SystemParametersInfo(SPI_GETWHEELSCROLLLINES, 0,
-		    &mouse_scroll_lines, 0);
+	    mouse_set_vert_scroll_step(mouse_vertical_scroll_step());
 	    break;
-	case SPI_GETWHEELSCROLLCHARS:
-	    SystemParametersInfo(SPI_GETWHEELSCROLLCHARS, 0,
-		    &mouse_scroll_chars, 0);
+	case SPI_SETWHEELSCROLLCHARS:
+	    mouse_set_hor_scroll_step(mouse_horizontal_scroll_step());
 	    break;
 	case SPI_SETNONCLIENTMETRICS:
 	    set_tabline_font();

src/mouse.c

@@ -13,6 +13,25 @@
 
 #include "vim.h"
 
+/*
+ * Horiziontal and vertical steps used when scrolling.
+ * When negative scroll by a whole page.
+ */
+static long mouse_hor_step = 6;
+static long mouse_vert_step = 3;
+
+    void
+mouse_set_vert_scroll_step(long step)
+{
+    mouse_vert_step = step;
+}
+
+    void
+mouse_set_hor_scroll_step(long step)
+{
+    mouse_hor_step = step;
+}
+
 #ifdef CHECK_DOUBLE_CLICK
 /*
  * Return the duration from t1 to t2 in milliseconds.
@@ -1101,13 +1120,16 @@ ins_mousescroll(int dir)
     // Don't scroll the window in which completion is being done.
     if (!pum_visible() || curwin != old_curwin)
     {
+	long step;
+
 	if (dir == MSCR_DOWN || dir == MSCR_UP)
 	{
-	    if (mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))
-		scroll_redraw(dir,
-			(long)(curwin->w_botline - curwin->w_topline));
+	    if (mouse_vert_step < 0
+		    || mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))
+		step = (long)(curwin->w_botline - curwin->w_topline);
 	    else
-		scroll_redraw(dir, 3L);
+		step = mouse_vert_step;
+	    scroll_redraw(dir, step);
 # ifdef FEAT_PROP_POPUP
 	if (WIN_IS_POPUP(curwin))
 	    popup_set_firstline(curwin);
@@ -1116,10 +1138,13 @@ ins_mousescroll(int dir)
 #ifdef FEAT_GUI
 	else
 	{
-	    int val, step = 6;
+	    int val;
 
-	    if (mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))
+	    if (mouse_hor_step < 0
+		    || mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))
 		step = curwin->w_width;
+	    else
+		step = mouse_hor_step;
 	    val = curwin->w_leftcol + (dir == MSCR_RIGHT ? -step : step);
 	    if (val < 0)
 		val = 0;
@@ -2005,8 +2030,9 @@ retnomove:
 }
 
 /*
- * Mouse scroll wheel: Default action is to scroll three lines, or one page
- * when Shift or Ctrl is used.
+ * Mouse scroll wheel: Default action is to scroll mouse_vert_step lines (or
+ * mouse_hor_step, depending on the scroll direction), or one page when Shift or
+ * Ctrl is used.
  * K_MOUSEUP (cap->arg == 1) or K_MOUSEDOWN (cap->arg == 0) or
  * K_MOUSELEFT (cap->arg == -1) or K_MOUSERIGHT (cap->arg == -2)
  */
@@ -2033,7 +2059,6 @@ nv_mousescroll(cmdarg_T *cap)
 	curwin = wp;
 	curbuf = curwin->w_buffer;
     }
-
     if (cap->arg == MSCR_UP || cap->arg == MSCR_DOWN)
     {
 # ifdef FEAT_TERMINAL
@@ -2043,21 +2068,21 @@ nv_mousescroll(cmdarg_T *cap)
 	    send_keys_to_term(curbuf->b_term, cap->cmdchar, mod_mask, FALSE);
 	else
 # endif
-	if (mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))
+	if (mouse_vert_step < 0 || mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))
 	{
 	    (void)onepage(cap->arg ? FORWARD : BACKWARD, 1L);
 	}
 	else
 	{
 	    // Don't scroll more than half the window height.
-	    if (curwin->w_height < 6)
+	    if (curwin->w_height < mouse_vert_step * 2)
 	    {
 		cap->count1 = curwin->w_height / 2;
 		if (cap->count1 == 0)
 		    cap->count1 = 1;
 	    }
 	    else
-		cap->count1 = 3;
+		cap->count1 = mouse_vert_step;
 	    cap->count0 = cap->count1;
 	    nv_scroll_line(cap);
 	}
@@ -2072,10 +2097,13 @@ nv_mousescroll(cmdarg_T *cap)
 	// Horizontal scroll - only allowed when 'wrap' is disabled
 	if (!curwin->w_p_wrap)
 	{
-	    int val, step = 6;
+	    int val, step;
 
-	    if (mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))
+	    if (mouse_hor_step < 0
+		    || mod_mask & (MOD_MASK_SHIFT | MOD_MASK_CTRL))
 		step = curwin->w_width;
+	    else
+		step = mouse_hor_step;
 	    val = curwin->w_leftcol + (cap->arg == MSCR_RIGHT ? -step : +step);
 	    if (val < 0)
 		val = 0;

src/proto/mouse.pro

@@ -1,4 +1,6 @@
 /* mouse.c */
+void mouse_set_vert_scroll_step(long step);
+void mouse_set_hor_scroll_step(long step);
 int do_mouse(oparg_T *oap, int c, int dir, long count, int fixindent);
 void ins_mouse(int c);
 void ins_mousescroll(int dir);

src/testdir/test_gui.vim

@@ -997,6 +997,7 @@ func Test_gui_mouse_event()
   call assert_equal(['one two abc three', 'four five posix'], getline(1, '$'))
 
   %d _
+  set scrolloff=0
   call setline(1, range(1, 100))
   " scroll up
   let args = #{button: 0x200, row: 2, col: 1, multiclick: 0, modifiers: 0}
@@ -1012,6 +1013,7 @@ func Test_gui_mouse_event()
   call test_gui_event('mouse', args)
   call feedkeys("H", 'Lx!')
   call assert_equal(4, line('.'))
+  set scrolloff&
 
   %d _
   set nowrap

src/testing.c

@@ -1393,6 +1393,11 @@ test_gui_mouse_event(dict_T *args)
 	repeated_click = (int)dict_get_number(args, (char_u *)"multiclick");
 	mods = (int)dict_get_number(args, (char_u *)"modifiers");
 
+	// Reset the scroll values to known values.
+	// XXX: Remove this when/if the scroll step is made configurable.
+	mouse_set_hor_scroll_step(6);
+	mouse_set_vert_scroll_step(3);
+
 	gui_send_mouse_event(button, TEXT_X(col - 1), TEXT_Y(row - 1),
 							repeated_click, mods);
     }

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4902,
 /**/
     4901,
 /**/