Args
ARG(1) is a handle to any parent window of the window upon which the user clicked.
ARG(2) indicates which part of the window the mouse is over. It the same value as what you return from your NCHITTEST event handler.
Return
You must return one of the following values:
Value Meaning 1 Activate the window and cause subsequent mouse-related events to occur (such as LBUTTONDOWN if he clicked the left mouse button). 2 Activate the window, but don't cause subsequent mouse-related events to occur. 3 Do not activate the window. Leave it inactive, but cause subsequent mouse-related events to occur. 4 Do not activate the window, and don't cause subsequent mouse-related events to occur.
Notes
See also ACTIVATE.