1. override
double snapScrollOffset(double newScrollOffset)

Returns the snapped offset closest to the given scroll offset.

Source 

@override
double snapScrollOffset(double newScrollOffset) {
  final double previousItemOffset = newScrollOffset.floorToDouble();
  final double nextItemOffset = newScrollOffset.ceilToDouble();
  return (newScrollOffset - previousItemOffset < 0.5 ? previousItemOffset : nextItemOffset)
    .clamp(scrollBehavior.minScrollOffset, scrollBehavior.maxScrollOffset);
}