MiniGUI/examples/msg_sizechanged.c

/**
 * The handler makes sure that the client area always
 * is _WIDTH wide and _HEIGHT high.
 */
        case MSG_SIZECHANGED:
        {
            RECT* rcClient = (RECT*)lParam;
            rcClient->right = rcClient->left + _WIDTH;
            rcClient->bottom = rcClient->top + _HEIGHT;
            return 1;
        }