Whether the semantics of the modal barrier are included in the semantics tree.
The modal barrier is the scrim that is rendered behind each route, which generally prevents the user from interacting with the route below the current route, and normally partially obscures such routes.
If semanticsDismissible is true, then modal barrier semantics are included in the semantics tree.
If semanticsDismissible is false, then modal barrier semantics are excluded from the semantics tree and tapping on the modal barrier has no effect.
It is safe to use
navigator.context to look up inherited
widgets here, because the Navigator calls
changedExternalState whenever its dependencies change, and
changedExternalState causes the modal barrier to rebuild.
bool get semanticsDismissible => true;