code: public static <T, C extends Cell<T, ?>> VirtualFlow<T, C> createVertical(
              ObservableList<T> items,
              Function<? super T, ? extends C> cellFactory,
              Gravity gravity) {
          return new VirtualFlow<>(items, cellFactory, new VerticalHelper(), gravity);
      }
docs: Creates a viewport that lays out content vertically from top to bottom