JWindowを半透明の滑らかな図形で切り抜きする

Shape shape = new RoundRectangle2D.Float(0f, 0f, 240f, 64f, 32f, 32f);

JButton button2 = new JButton("not use Window#setShape(...)");
button2.addActionListener(e -> {
  JWindow window = new JWindow();
  window.setBackground(new Color(0x0, true));
  window.getContentPane().add(makePanel(shape));
  window.pack();
  window.setLocationRelativeTo(((AbstractButton) e.getSource()).getRootPane());
  window.setVisible(true);
});

// ...

private Component makePanel(Shape shape) {
  JPanel panel = new JPanel(new BorderLayout()) {
    @Override public Dimension getPreferredSize() {
      return shape.getBounds().getSize();
    }

    @Override protected void paintComponent(Graphics g) {
      Graphics2D g2 = (Graphics2D) g.create();
      g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC, .5f));
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setPaint(Color.RED);
      g2.fill(shape);
      g2.dispose();
      super.paintComponent(g);
    }
  };
  panel.setOpaque(false);
  panel.setBorder(BorderFactory.createEmptyBorder(8, 8, 8, 8));
  // ...
  return panel;
}