JProgressBarでページのスクロールした距離を表示する

JScrollPane scroll = new JScrollPane(new JTextArea(buf.toString()));
BoundedRangeModel model = scroll.getVerticalScrollBar().getModel();
JProgressBar progress = new ScrollIndicator(model);
scroll.setColumnHeaderView(progress);
// ...

class ScrollIndicator extends JProgressBar {
  protected ScrollIndicator(BoundedRangeModel model) {
    super(model);
  }

  @Override public void updateUI() {
    super.updateUI();
    setUI(new ScrollIndicatorUI());
    setBorder(BorderFactory.createEmptyBorder());
  }

  @Override public double getPercentComplete() {
    BoundedRangeModel m = getModel();
    int max = m.getMaximum();
    int min = m.getMinimum();
    double maxExtent = Math.max(max - m.getExtent() - min, 0d);
    return (m.getValue() - min) / maxExtent;
  }

  @Override public Dimension getPreferredSize() {
    Dimension d = super.getPreferredSize();
    if (getOrientation() == HORIZONTAL) {
      d.height = 4;
    } else {
      d.width = 4;
    }
    return d;
  }
}

class ScrollIndicatorUI extends BasicProgressBarUI {
  @Override public void paintDeterminate(Graphics g, JComponent c) {
    Insets b = progressBar.getInsets();
    Rectangle r = SwingUtilities.calculateInnerArea(progressBar, null);
    if (!r.isEmpty()) {
      int amountFull = getAmountFull(b, r.width, r.height); // + extent;
      Graphics2D g2 = (Graphics2D) g.create();
      g2.setColor(UIManager.getColor("ProgressBar.foreground"));
      if (progressBar.getOrientation() == SwingConstants.HORIZONTAL) {
        g2.fillRect(r.x, r.y, amountFull, r.height);
      } else { // VERTICAL
        // Draw progress from top to bottom
        g2.fillRect(r.x, r.y, r.width, amountFull);
        // Draw progress from bottom to top
        // g2.fillRect(r.x, r.y + r.height - amountFull, r.width, amountFull);
      }
      // Deal with possible text painting
      if (progressBar.isStringPainted()) {
        paintString(g2, r.x, r.y, r.width, r.height, amountFull, b);
      }
      g2.dispose();
    }
  }
}