The WM_NCHITTEST Message:
Whenever a mouse event occurs, the system sends a WM_NCHITTEST message to either the window that contains the cursor hot spot or the window that has captured the mouse. The system uses this message to determine whether to send a client area or nonclient area mouse message. An application that must receive mouse movement and mouse button messages must pass the WM_NCHITTEST message to the DefWindowProc function

HTTRANSPARENT: In a window currently covered by another window in the same thread (the message will be sent to underlying windows in the same thread until one of them returns a code that is not HTTRANSPARENT).