// Only call this from places that will definitely trigger a rebuild.
  void _updatePosition() {
    _configuration = ScrollConfiguration.of(context);
    _physics = _configuration.getScrollPhysics(context);
    if (widget.physics != null)
      _physics = widget.physics.applyTo(_physics);
    final ScrollController controller = widget.controller;
    final ScrollPosition oldPosition = position;
    if (oldPosition != null) {
      controller?.detach(oldPosition);
      scheduleMicrotask(oldPosition.dispose);
    }
    _position = controller?.createScrollPosition(_physics, this, oldPosition)
      ?? ScrollPositionWithSingleContext(physics: _physics, context: this, oldPosition: oldPosition);
    assert(position != null);
    controller?.attach(position);
  }