Possible values are described in the following list:
VK_BUTTON1
Release of button 1 ends the drag.
VK_BUTTON2
Release of button 2 ends the drag.
VK_BUTTON3
Release of button 3 ends the drag.
VK_ENDDRAG
Release of the system-defined direct
manipulation button ends the drag. This is the recommended value if the
DrgDrag function call is invoked in response
to a WM_BEGINDRAG message.