visible property
Switches between showing the sliver or hiding it.
The maintain flags should be set to the same values regardless of the
state of the visible property, otherwise they will not operate correctly
(specifically, the state will be lost regardless of the state of
maintainState whenever any of the maintain flags are changed, since
doing so will result in a subtree shape change).
Unless maintainState is set, the sliver subtree will be disposed
(removed from the tree) while hidden.
Implementation
final bool visible;