WM_NCMBUTTONUP    
The
WM_NCMBUTTONUP message is posted when the user releases the middle mouse button
while the cursor is within the nonclient area of a window. This message is
posted to the window that contains the cursor. If a window has captured the
mouse, this message is not posted. 
WM_NCMBUTTONUP 
nHittest = (INT) wParam;    // hit-test value 
pts = MAKEPOINTS(lParam);   // position of cursor 
 
Parameters
nHittest
Value of wParam.
Specifies the hit-test value returned by the DefWindowProc function as a result of
processing the WM_NCHITTEST message. For a list of hit-test values, see WM_NCHITTEST. 
pts
Value of lParam.
Specifies a POINTS
structure that contains the x- and y-coordinates of the cursor. The coordinates
are relative to the upper-left corner of the screen. 
Return Values
If an
application processes this message, it should return zero. 
Remarks
An
application can use the MAKEPOINTS macro to convert the lParam
parameter to a POINTS structure. 
If it is
appropriate to do so, the system sends the WM_SYSCOMMAND message to the window.
See Also