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        viewable 
      
- 
        A window is viewable if it and all of its ancestors are mapped.
        This does not imply that any portion of the window is actually
        visible. Graphics requests can be performed on a window when it is not
        viewable, but output will not be retained unless the server is
        maintaining backing store. 
      
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        visible 
      
- 
        A region of a window is visible if someone looking at the
        screen can actually see it; that is, the window is viewable and the
        region is not occluded by any other window.