This message is posted when the user presses the touch screen in the client area of a window or presses the left mouse button while the cursor is located within the client area of a window. If another window does not capture the mouse or stylus event, the OS posts the message to the window beneath the stylus or mouse. If another window does capture the mouse or stylus, the OS posts the message to the window that has the capture.
WM_LBUTTONDOWN fwKeys = wParam; xPos = LOWORD(lParam); yPos = HIWORD(lParam);
Indicates the mouse buttons and keys that the user pressed. The following table shows the possible values, which can be combined.
The user pressed the CTRL key.
The user pressed the left mouse button.
The left mouse button is up.
The user pressed the right mouse button.
The user pressed the SHIFT key.
Value of the low-order word of lParam. Specifies the x-coordinate of the cursor, relative to the upper-left corner of the client area.
Value of the high-order word of lParam. Specifies the y-coordinate of the cursor, relative to the upper-left corner of the client area.