static <A extends JComponent> A setOpaqueBackground_recursive(Color color, A a) {
  setOpaqueBackground(color, a);
  if (a instanceof JPanel) {
    ((JPanel) a).setOpaque(false);
    for (Component c : getSwingChildren(a))
      if (c cast JComponent)
        setOpaqueBackground_recursive(color, c);
  }
  ret a;
}