isActive method
Whether or not the given TreeViewNode is enclosed within its parent TreeViewNode.
If the TreeViewNode.parent isExpanded, or this is a root node, the given node is active and this method will return true. This does not reflect whether or not the node is visible in the Viewport.
Implementation
bool isActive(TreeViewNode<Object?> node) {
assert(_state != null);
return _state!.isActive(node);
}