The position of the child relative to the zero scroll offset.
The number of pixels from from the zero scroll offset of the parent sliver (the line at which its SliverConstraints.scrollOffset is zero) to the side of the child closest to that offset. A layoutOffset can be null when it cannot be determined. The value will be set after layout.
In a typical list, this does not change as the parent is scrolled.
Defaults to null.