anchor.diff:    bool get hasVisualOverflow => _hasVisualOverflow;
   
     @override
  -  void updateOutOfBandData(GrowthDirection growthDirection, SliverGeometry childLayoutGeometry) {
  +  void updateOutOfBandData(
  +      GrowthDirection growthDirection, SliverGeometry childLayoutGeometry) {
       switch (growthDirection) {
         case GrowthDirection.forward:
           _maxScrollExtent += childLayoutGeometry.scrollExtent;
  
positive.code:   /// bar) can tag its [SemanticsNode] with [excludeFromScrolling] to indicate
    /// that it should no longer be considered for semantic actions related to
    /// scrolling.
    static const SemanticsTag excludeFromScrolling = SemanticsTag('RenderViewport.excludeFromScrolling');
  
    @override
    void setupParentData(RenderObject child) {
  
negative.code:     child = lastChild;
      while (true) {
        yield child!;
        if (child == center) return;
        child = childBefore(child);
      }
    }
  
    @override
    Iterable<RenderSliver> get childrenInHitTestOrder sync* {
      if (firstChild == null) return;
      RenderSliver? child = center;
      while (child != null) {
        yield child;
  