The WM_NCMBUTTONDOWN message is posted when the user presses 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_NCMBUTTONDOWN
nHittest = (INT) wParam; // hit-test value
pts = MAKEPOINTS(lParam); // position of cursor
If an application processes this message, it should return zero.
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.
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Unsupported.
Header: Declared in winuser.h.
Mouse Input Overview, Mouse Input Messages, DefWindowProc, MAKEPOINTS, POINTS, WM_NCHITTEST, WM_NCMBUTTONDBLCLK, WM_NCMBUTTONUP, WM_SYSCOMMAND