Windows uses the WM_NCHITTEST message to generate all other mouse messages. The idea of messages giving birth to other messages is common in Windows. Let's take an example. As you know, if you double-click the system menu box of a Windows program, the program will be terminated. The double-click generates a series of WM_NCHITTEST messages. Because the mouse is positioned over the system menu box, DefWindowProc returns a value of HTSYSMENU and Windows puts a WM_NCLBUTTONDBLCLK message in the message queue with wParam equal to HTSYSMENU.
The window procedure usually passes that mouse message to DefWindowProc. When DefWindowProc receives the WM_NCLBUTTONDBLCLK message with wParam equal to HTSYSMENU, it puts a WM_SYSCOMMAND message with wParam equal to SC_CLOSE in the message queue. (This WM_SYSCOMMAND message is also generated when a user selects Close from the system menu box.) Again, the window procedure usually passes that message to DefWindowProc. DefWindowProc processes the message by sending a WM_CLOSE message to the window procedure.
If a program wants to require confirmation from a user before terminating, the window procedure can trap WM_CLOSE. Otherwise, DefWindowProc processes WM_CLOSE by calling the DestroyWindow function. Among other actions, DestroyWindow sends a WM_DESTROY message to the window procedure. Normally, a window procedure processes WM_DESTROY with the code:
case WM_DESTROY :
PostQuitMessage (0) ;
return 0 ;
The PostQuitMessage causes Windows to place a WM_QUIT message in the message queue. This message never reaches the window procedure because it causes GetMessage to return 0, which terminates the message loop and the program.