if (child.isFixedDescendant()) { setFixedDescendant(true); }
    public void addChild(Box child) {        if (child == null) {            throw new NullPointerException("trying to add null child");        }        child.setParent(this);        boxes.add(child);        if (child.isChildrenExceedBounds()) {            setChildrenExceedBounds(true);        }    }