To make your application process timer messages, add a WM_TIMER case to the window procedure, as follows:
case WM_LBUTTONDBLCLK:
    wsprintf(ButtonText, "WM_LBUTTONDBLCLK: %x, %d, %d",
        wParam, LOWORD(lParam), HIWORD(lParam));
    InvalidateRect(hWnd, &rectButton, TRUE);
    break;