anchor.diff:    ///
     /// For example, if the sliver wishes to paint visually before its layout
     /// position, the [paintOrigin] is negative. The coordinate system this sliver
  -  /// uses for painting is relative to this [paintOrigin].
  +  /// uses for painting is relative to this [paintOrigin]. In other words,
  +  /// when [RenderSliver.paint] is called, the (0, 0) position of the [Offset]
  +  /// given to it is at this [paintOrigin].
  +  ///
  +  /// The coordinate system used for the [paintOrigin] itself is relative
  +  /// to the start of this sliver's layout position rather than relative to
  +  /// its current position on the viewport. In other words, in a typical
  +  /// scrolling scenario, [paintOrigin] remains constant at 0.0 rather than
  +  /// tracking from 0.0 to [SliverConstraints.viewportMainAxisExtent] as the
  +  /// sliver scrolls past the viewport.
     ///
     /// This value does not affect the layout of subsequent slivers. The next
     /// sliver is still placed at [layoutExtent] after this sliver's layout
  
positive.code:   /// [SliverConstraints.scrollOffset] is zero.
    ///
    /// This must be between zero and [paintExtent]. It defaults to [paintExtent].
    final double layoutExtent;
  
    /// The (estimated) total paint extent that this sliver would be able to
  
negative.code:   /// 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 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].
  