void _updatePosition() {
    _configuration = widget.scrollBehavior ?? ScrollConfiguration.of(context);
    _physics = _configuration.getScrollPhysics(context);
    if (widget.physics != null) {
      _physics = widget.physics!.applyTo(_physics);
    } else if (widget.scrollBehavior != null) {
      _physics = widget.scrollBehavior!.getScrollPhysics(context).applyTo(_physics);
    }
    final ScrollPosition? oldPosition = _position;
    if (oldPosition != null) {
      _effectiveScrollController.detach(oldPosition);   

    // **********************************************
     // **********************************************
    //   During this period, the scrollcontroller is attached
    //    with two positions, which will trigger a bug.
    // **********************************************
   // **********************************************

      // It's important that we not dispose the old position until after the
      // viewport has had a chance to unregister its listeners from the old
      // position. So, schedule a microtask to do it.
      scheduleMicrotask(oldPosition.dispose);
    }

    _position = _effectiveScrollController.createScrollPosition(_physics!, this, oldPosition);
    assert(_position != null);
    _effectiveScrollController.attach(position);
  }