The preferred smallest size the scrollbar thumb can shrink to when viewport is overscrolled.
When overscrolling, the size of the scrollbar's thumb may shrink to a smaller size than minOverscrollLength to fit in the available paint area (e.g., when minOverscrollLength is greater than ScrollMetrics.viewportDimension and mainAxisMargin combined).
Overscrolling can be made possible by setting the
child Widget to a
BouncingScrollPhysics, which is a special
ScrollPhysics that allows overscrolling.
final double? minOverscrollLength;