The preferred smallest size the scrollbar thumb can shrink to when the total scrollable extent is large, the current visible viewport is small, and the viewport is not overscrolled.
The size of the scrollbar's thumb may shrink to a smaller size than minThumbLength to fit in the available paint area (e.g., when minThumbLength is greater than ScrollMetrics.viewportDimension and mainAxisMargin combined).
Mustn't be null and the value has to be greater or equal to minOverscrollLength, which in turn is >= 0. Defaults to 18.0.
final double minThumbLength;