Flattens a tree into a visible row list. A node's children are included only when its id is in expandedIds.
expandedIds
The root-level TaskNode instances of the tree.
Set of task IDs whose children should be rendered.
A depth-first flattened array of visible TaskNode items.
Flattens a tree into a visible row list. A node's children are included only when its id is in
expandedIds.