OverscrollWhenScrollableBehavior({double contentExtent: double.INFINITY, double containerExtent: 0.0, double minScrollOffset: 0.0, @required TargetPlatform platform })

Creates a scroll behavior that allows overscrolling only when some amount of scrolling is already possible.

Source 

OverscrollWhenScrollableBehavior({
  double contentExtent: double.INFINITY,
  double containerExtent: 0.0,
  double minScrollOffset: 0.0,
  @required TargetPlatform platform
}) : super(
  contentExtent: contentExtent,
  containerExtent: containerExtent,
  minScrollOffset: minScrollOffset,
  platform: platform
);