layoutExtent property Null safety
The distance from the first visible part of this sliver to the first visible part of the next sliver, assuming the next sliver's 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 the 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;