position property
final
By default, the child will be rendered right below (if the parent is Column)
the widget which is defined alongside the OverlayContainer.
It would appear as though the Overlay is inside its parent
but in reality it would be outside and above
the original widget hierarchy.
It's position can be altered and the overlay can
be moved to any part of the screen by supplying a position
argument.
Implementation
final OverlayContainerPosition position;