crossAxisPosition property

The distance to the hit point in the axis opposite the SliverConstraints.axis.

If the cross axis is horizontal (i.e. the SliverConstraints.axisDirection is either AxisDirection.down or AxisDirection.up), then the crossAxisPosition is a distance from the left edge of the sliver. If the cross axis is vertical (i.e. the SliverConstraints.axisDirection is either AxisDirection.right or AxisDirection.left), then the crossAxisPosition is a distance from the top edge of the sliver.

This is always a distance from the left or top of the parent, never a distance from the right or bottom.