Swing/AutoScrollOnFocus のバックアップ(No.4)

Box box = Box.createVerticalBox();
box.setBorder(BorderFactory.createEmptyBorder(5, 5, 5, 5));
IntStream.range(0, 20).forEach(i -> {
  box.add(new JTextField("test" + i));
  box.add(Box.createVerticalStrut(5));
});
box.add(Box.createVerticalGlue());
box.setFocusCycleRoot(true);
box.setFocusTraversalPolicy(new LayoutFocusTraversalPolicy() {
  @Override public Component getComponentAfter(Container focusCycleRoot, Component aComponent) {
    Component c = super.getComponentAfter(focusCycleRoot, aComponent);
    if (focusCycleRoot instanceof JComponent) {
      ((JComponent) focusCycleRoot).scrollRectToVisible(c.getBounds());
    }
    return c;
  }
  @Override public Component getComponentBefore(Container focusCycleRoot, Component aComponent) {
    Component c = super.getComponentBefore(focusCycleRoot, aComponent);
    if (focusCycleRoot instanceof JComponent) {
      ((JComponent) focusCycleRoot).scrollRectToVisible(c.getBounds());
    }
    return c;
  }
});
add(new JScrollPane(box));