isVisible method
Whether key is currently in the flattened visible order — i.e.
every ancestor (if any) is expanded AND the node itself exists.
O(1) via the visible-order buffer's nid-indexed reverse lookup. Useful for the render object to distinguish "truly hidden" from "ghost-rendered after a visible→hidden reparent."
Implementation
bool isVisible(TKey key) {
_ensureVisibleOrder();
return _order.contains(key);
}