JViewportにBorderを設定する

JTextArea textArea1 = new JTextArea("JTextArea#setMargin(Insets)\n\n" + INIT_TXT);
textArea1.setMargin(new Insets(5, 5, 5, 5));
JScrollPane scroll1 = new JScrollPane(textArea1);

JTextArea textArea2 = new JTextArea("JScrollPane#setViewportBorder(...)\n\n" + INIT_TXT);
textArea2.setMargin(new Insets(0, 0, 0, 1));
JScrollPane scroll2 = new JScrollPane(textArea2) {
  @Override public void updateUI() {
    setViewportBorder(null);
    super.updateUI();
    EventQueue.invokeLater(() -> {
      Component c = getViewport().getView();
      setViewportBorder(BorderFactory.createLineBorder(c.getBackground(), 5));
    });
};
// scroll2.setViewportBorder(BorderFactory.createLineBorder(textArea2.getBackground(), 5));