WM_VSCROLL

  This message is sent to a window when its vertical scroll bar is clicked.

Parameters
  wParam can be one of the following :

SB_LINEDOWN -  user  clicked  on  the  bottom  arrow  of  the  scrollbar.
SB_LINEUP -  user  clicked  on  the  top  arrow  of  the  scrollbar.
SB_PAGEDOWN -  user  clicked  below  the  thumb  position.
SB_PAGEUP -  user  clicked  above  the  thumb  position.
SB_THUMBTRACK -  user  dragged  the  thumb  to  a  new  position.
SB_THUMBPOSITION -  the  user  finished  dragging  the  thumb
SB_ENDSCROLL -  the  user  released  the  mouse  button  on  one  of  the
                     scrollbar arrows

  HIWORD(lParam) is the handle of the scrollbar.
  LOWORD(lParam) is the current thumb position if wParam is SB_THUMBTRACK,
SB_THUMBPOSITION, or  SB_ENDSCROLL,  and  for  all  other  values  of  wParam,
is  not used and is set to 0

.