anchor.diff:    /// the earliest visible part of this sliver in the [AxisDirection].
     ///
     /// For example, if [AxisDirection] is [AxisDirection.down], then this is the
  -  /// scroll offset at the top of the visible portion of the sliver.
  +  /// scroll offset at the top of the visible portion of the sliver or
  +  /// equivalently the amount the top of the sliver has been scrolled past the
  +  /// top of the viewport.
  +  ///
  +  /// This value is typically used to compute whether this sliver should still
  +  /// protrude into the viewport via [SliverGeometry.paintExtent] and
  +  /// [SliverGeometry.layoutExtent] considering how far the beginning of the
  +  /// sliver is above the beginning of the viewport.
  +  ///
  +  /// For slivers whose top is not past the top of the viewport, the
  +  /// [scrollOffset] is `0` when [AxisDirection] is [AxisDirection.down]. This
  +  /// includes all the slivers that are below the bottom of the viewport.
  +  ///
  +  /// [SliverConstraints.remainingPaintExtent] is typically used to accomplish
  +  /// the same goal of computing whether scrolled out slivers should still
  +  /// partially 'protrude in' from the bottom of the viewport.
     ///
     /// Whether this corresponds to the beginning or the end of the sliver's
     /// contents depends on the [growthDirection].
  
positive.code:   /// If this is non-zero after [RenderSliver.performLayout] returns, the scroll
    /// offset will be adjusted by the parent and then the entire layout of the
    /// parent will be rerun.
    ///
    /// If the parent is also a [RenderSliver], it must propagate this value
    /// in its own [RenderSliver.geometry] property until a viewport which adjusts
  
negative.code:   /// [SliverConstraints.scrollOffset] is zero.
    ///
    /// This must be between zero and [paintExtent]. It defaults to [paintExtent].
    ///
    /// This value is typically 0 when outside of the viewport and grows or
    /// shrinks from 0 or to 0 as the sliver is being scrolled into and out of the
    /// viewport unless then sliver wants to achieve a special effect and push
    /// down the layout start position of subsequent slivers before the sliver is
    /// even scrolled into the viewport.
    final double layoutExtent;
  
    /// The (estimated) total paint extent that this sliver would be able to
  