WM_KILLFOCUS
hwndGetFocus = (HWND) lParam; /* handle of window receiving focus */
The WM_KILLFOCUS message is sent immediately before a window loses the input focus.
hwndGetFocus
Value of wParam. Identifies the window that receives the input focus. (This parameter may be NULL.)
An application should return zero if it processes this message.
If an application is displaying a caret, the caret should be destroyed at this point.