GetFirstVisibleIndex returns same values before and after scrolling

[PavelTurk]

This is my test code:

public class FlawTest extends Application {

    private static class FlowRow implements Cell<Integer, Node> {

        private Integer rowIndex;

        private final Label label = new Label();

        private final HBox hBox = new HBox(label);

        public FlowRow(Integer rowIndex) {
            this.rowIndex = rowIndex;
            label.setText(String.valueOf(rowIndex));
        }

        @Override
        public void updateItem(Integer item) {
            rowIndex = item;
            if (item == null) {
                label.setText("");
            } else {
                label.setText(String.valueOf(rowIndex));
            }
        }

        @Override
        public Node getNode() {
            return hBox;
        }

        @Override
        public boolean isReusable() {
            return true;
        }
    }

    private VirtualFlow<Integer, FlowRow> flow;

    private ObservableList<Integer> rows;

    @Override
    public void start(Stage primaryStage) throws Exception {
        List<Integer> list = new ArrayList<>();
        for (var i = 0; i < 100000; i++) {
            list.add(i);
        }
        rows = FXCollections.observableList(list);

        flow = VirtualFlow.createVertical(rows, row -> new FlowRow(row));
		VirtualizedScrollPane<VirtualFlow<Integer, FlowRow>> scrollPane = new VirtualizedScrollPane<>(flow);
        VBox.setVgrow(scrollPane, Priority.ALWAYS);

        var scrollButton = new Button("Scroll");
        scrollButton.setOnAction(e -> {
            System.out.println("Before scrolling first cell index: " + flow.getFirstVisibleIndex());
            flow.scrollYBy(flow.getHeight());
            System.out.println("After scrolling first cell index: " + flow.getFirstVisibleIndex());
        });

        VBox root = new VBox(scrollPane, new HBox(scrollButton));
        Scene scene = new Scene(root, 400, 300);
        primaryStage.setTitle("Flaw Test");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }

}

If you push button you will get the following output:

Before scrolling first cell index: 0
After scrolling first cell index: 0
Before scrolling first cell index: 17
After scrolling first cell index: 17

I tried:

scrollButton.setOnAction(e -> {
    System.out.println("Before scrolling first cell index: " + flow.getFirstVisibleIndex());
    flow.scrollYBy(flow.getHeight());
    Platform.runLater(() -> System.out.println("After scrolling first cell index: " + flow.getFirstVisibleIndex()));
});

but the result was the same.