JTabbedPaneで選択されているタブの幅のみ拡張する

public static void updateTabWidth(JTabbedPane tabs) {
  int tp = tabs.getTabPlacement();
  if (tp == JTabbedPane.LEFT || tp == JTabbedPane.RIGHT) {
    return;
  }
  int sidx = tabs.getSelectedIndex();
  for (int i = 0; i < tabs.getTabCount(); i++) {
    Component c = tabs.getTabComponentAt(i);
    if (c instanceof ShrinkLabel) {
      ((ShrinkLabel) c).setSelected(i == sidx);
    }
  }
}

class ShrinkLabel extends JLabel {
  private boolean isSelected;
  protected ShrinkLabel(String title, Icon icon) {
    super(title, icon, SwingConstants.LEFT);
  }

  @Override public Dimension getPreferredSize() {
    Dimension d = super.getPreferredSize();
    if (!isSelected) {
      d.width = 20;
    }
    return d;
  }

  public void setSelected(boolean active) {
    this.isSelected = active;
  }

  public boolean isSelected() {
    return isSelected;
  }
}