activateFocused method
Toggle / activate the focused row: folders expand, leaves select. Returns the activated leaf node (for the host to open), or null.
Implementation
TreeNode<T>? activateFocused() {
final id = focused;
if (id == null) return null;
final n = node(id);
if (n == null) return null;
if (n.isFolder) {
toggle(id);
return null;
}
select(id);
return n;
}