title: how to invalidate parent window without sending wm_paint to child window?
body: <p>the parent and the child window is in the same size.
  and the the parent listens to the child's repainting
  when child repainting, the parent repainting.
  so I cannot use invalidate to clean the parent window,
  cos this will send wm_paint to child window, then a endless cycle.</p>
  
  <p>how can i clean up parent widnow without use invalidateRect,invalidateRgn and so on.
  or, how can i invalidate parent without send wm_paint to child window?</p>
  
  <p>thx!</p>