If you put a MapPanel in a window and move the window, the map erroneously uses cached offsets. An app developer could listen for move on the window and call updateSize on the MapPanel's map, but this requires special knowledge of OpenLayers. It also requires a reference to the MapPanel at least - which may not always be available when configuring the owner container.
So, a nice alternative is to check if the MapPanel has an ownerCt after it has been rendered. The MapPanel can listen for move and call updateSize on the map.