WM_SETFOCUS
hwndLoseFocus = (HWND) wParam; /* handle of window losing focus */
The WM_SETFOCUS message is sent to a window after it has gained the keyboard focus.
hwndLoseFocus
Value of wParam. Identifies the window that has lost the keyboard focus (may be NULL).
An application should return zero if it processes this message.
To display a caret, an application should call the appropriate caret functions at this point.
SetFocus, WM_SETFOCUS