The amount of currently visible visual space that was taken by the sliver to render the subset of the sliver that covers all or part of the SliverConstraints.remainingPaintExtent in the current viewport.
This value does not affect how the next sliver is positioned. In other words, if this value was 100 and layoutExtent was 0, typical slivers placed after it would end up drawing in the same 100 pixel space while painting.
This must be between zero and SliverConstraints.remainingPaintExtent.
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 paint even when scrolled away.
This contributes to the calculation for the next sliver's SliverConstraints.overlap.
final double paintExtent;