// component can be a custom-painted JComponent
static JWindow showAlwaysOnTopTranslucentWindow(Rect bounds, Color backgroundWithAlpha, JComponent component default null) {
  ret swing(() -> {
    new JWindow f;
    f.setAlwaysOnTop(true);
    int alpha = 128;
    f.setBackground(new Color(0,255,0, alpha));
    f.setBounds(toRectangle(bounds));
    add(f, component);
    f.setLocationRelativeTo(null);
    f.setVisible(true);
    ret f;
  });
}