_getActiveNodeIds(nodeId) {
    const activeNodeIds = this._activeNodeArraysById.get(nodeId);
    if (!activeNodeIds) throw new Error(`No such node ID ${nodeId}`);
    return activeNodeIds;
  }
Returns all the node IDs in a stack when a specific nodeId is at the top of the stack
(i.e. a stack's node ID and the node ID of all of its parents).

@param {number} nodeId
@return {Array<number>}