Earlier I discussed how the File Manager responded to mouse clicks and double-clicks. Obviously, the program must determine which file the user is pointing at with the mouse. This is called ”hit-testing.“ Just as DefWindowProc must do some hit-testing when processing WM_NCHITTEST messages, very often a window procedure must do some hit-testing within the client area. In general, hit-testing involves calculations using the x- and y-coordinates passed to your window procedure in the lParam parameter of the mouse message.