I have following problem, in my ListView the scrollbar fills 100% of its container so that it is impossible to use. This happens when the ListView has only one cell which is also bigger than its viewport. I wrote a small example to show it:

public class ExtendedScrollbarWrongHeight2 extends Application {
    private BorderPane borderPane;

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

    @Override
    public void start(Stage primaryStage) throws Exception {
        final Parent parent = createParent();
        final Scene scene = new Scene(parent, 640, 480);

        primaryStage.setScene(scene);
        primaryStage.show();
    }

    protected Parent createParent() {
        final ObservableList<String> items = FXCollections.observableArrayList();
        items.add("Scroll Down 1");
// items.add("Scroll Down 2"); // uncomment and bug is fixed

        final ListView<String> listView = new ListView<>();
        listView.setItems(items);
        listView.setCellFactory(param -> {
            return new CellCenter();
        });

        return listView;
    }

    private static class CellCenter extends ListCell<String> {
        private final Label label = new Label();

        @Override
        protected void updateItem(String item, boolean empty) {
            if (item == null || empty) {
                setGraphic(null);
            } else {
                label.setMaxHeight(USE_PREF_SIZE);
                label.setMaxWidth(USE_PREF_SIZE);
                label.setMinHeight(USE_PREF_SIZE);
                label.setMinWidth(USE_PREF_SIZE);
                label.setPrefHeight(700);
                label.setPrefWidth(640);
                label.setStyle("-fx-background-color: indianred; -fx-border-color: darkred;-fx-border-width: 1px;-fx-font-size: 40px;");
                label.setText("ScrollDown");

                setGraphic(label);
            }
        }
    }
}

When I add a second item to the ListView the scrollbar thumb has the correct size, but I also need it for single cells. I used the debugger and found a method updateScrollBarsAndCells() in VirtualFlow which is responsible for the calculation. It seems that the bug is known, see the comments in the last if case, but I found only one similar issue here (RT-29624) and I was not sure if it is the same problem.

if (recreate && (lengthBar.isVisible() || BehaviorSkinBase.IS_TOUCH_SUPPORTED)) {
            int numCellsVisibleOnScreen = 0;
            for (int i = 0, max = cells.size(); i < max; i++) {
                T cell = cells.get(i);
                if (cell != null && !cell.isEmpty()) {
                    sumCellLength += (isVertical ? cell.getHeight() : cell.getWidth());
                    if (sumCellLength > flowLength) {
                        break;
                    }

                    numCellsVisibleOnScreen++;
                }
            }

            lengthBar.setMax(1);
            if (numCellsVisibleOnScreen == 0 && cellCount == 1) {
                    // special case to help resolve RT-17701 and the case where we have
                // only a single row and it is bigger than the viewport
                lengthBar.setVisibleAmount(flowLength / sumCellLength);
            } else {
                lengthBar.setVisibleAmount(numCellsVisibleOnScreen / (float) cellCount);
            }
        }

Could you give me an alternative way how I can recalculate the scrollbar thumb size. I tried it by accessing the scrollbars via the ListView and it works only for selection. When I have two selected items and deselect one of it, it doesn't work. My helper method gets called before updateScrollBarsAndCells() and so the size will be overwritten.