The preferred smallest size the scrollbar 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 may shrink to a smaller size than minLength
to fit in the available paint area. E.g., when minLength is
double.infinity, it will not be respected if
mainAxisMargin are finite.
Mustn't be null and the value has to be within the range of 0 to minOverscrollLength, inclusive. Defaults to 18.0.
final double minLength