constructor(user, container, virtualDelegate, renderers, focusTrait, selectionTrait, anchorTrait, options) {
        super(user, container, virtualDelegate, renderers, options);
        this.focusTrait = focusTrait;
        this.selectionTrait = selectionTrait;
        this.anchorTrait = anchorTrait;
    }
We use this List subclass to restore selection and focus as nodes
get rendered in the list, possibly due to a node expand() call.