+static int
+can_scroll(struct scrollbar_instance* scrollbar)
+{
+ return scrollbar != NULL
+ && IsWindowVisible (SCROLLBAR_MSW_HANDLE (scrollbar))
+ && IsWindowEnabled (SCROLLBAR_MSW_HANDLE (scrollbar));
+}
+
+int
+mswindows_handle_mousewheel_event (Lisp_Object frame, int keys, int delta)
+{
+ int hasVertBar, hasHorzBar; /* Indicates presence of scroll bars */
+ unsigned wheelScrollLines = 0; /* Number of lines per wheel notch */
+
+ /* Find the currently selected window */
+ Lisp_Object win = FRAME_SELECTED_WINDOW (XFRAME (frame));
+ struct window* w = XWINDOW (win);
+ struct window_mirror* mirror = find_window_mirror (w);
+
+ /* Check that there is something to scroll */
+ hasVertBar = can_scroll (mirror->scrollbar_vertical_instance);
+ hasHorzBar = can_scroll (mirror->scrollbar_horizontal_instance);
+ if (!hasVertBar && !hasHorzBar)
+ return FALSE;
+
+ /* No support for panning and zooming, so ignore */
+ if (keys & (MK_SHIFT | MK_CONTROL))
+ return FALSE;
+
+ /* Get the number of lines per wheel delta */
+ SystemParametersInfo (SPI_GETWHEELSCROLLLINES, 0, &wheelScrollLines, 0);
+
+ /* Calculate the amount to scroll */
+ if (wheelScrollLines == WHEEL_PAGESCROLL)
+ {
+ /* Scroll by a page */
+ Lisp_Object function;
+ if (hasVertBar)
+ function = delta > 0 ? Qscrollbar_page_up : Qscrollbar_page_down;
+ else
+ function = delta > 0 ? Qscrollbar_page_left : Qscrollbar_page_right;
+ mswindows_enqueue_misc_user_event (frame, function, Fcons (win, Qnil));
+ }
+ else /* Scroll by a number of lines */
+ {
+ /* Calc the number of lines to scroll */
+ int toScroll = MulDiv (delta, wheelScrollLines, WHEEL_DELTA);
+
+ /* Do the scroll */
+ Lisp_Object function;
+ if (hasVertBar)
+ function = delta > 0 ? Qscrollbar_line_up : Qscrollbar_line_down;
+ else
+ function = delta > 0 ? Qscrollbar_char_left : Qscrollbar_char_right;
+ if (toScroll < 0)
+ toScroll = -toScroll;
+ while (toScroll--)
+ mswindows_enqueue_misc_user_event (frame, function, win);
+ }
+
+ return TRUE;
+}
+