userScrollDirection property Null safety
For example, if growthDirection is GrowthDirection.reverse and axisDirection is AxisDirection.down, then a ScrollDirection.forward means that the user is scrolling up, in the positive scrollOffset direction.
This is used by some slivers to determine how to react to a change in scroll offset. For example, RenderSliverFloatingPersistentHeader will only expand a floating app bar when the userScrollDirection is in the positive scroll offset direction.
final ScrollDirection userScrollDirection;