code: public static void makeWindowNonOpaque(Window window) {
          if (PlatformUtils.isJava6()) {
              // on non-mac platforms, try to use the facilities of Java 6 update 10.
              if (!PlatformUtils.isMac()) {
                  quietlyTryToMakeWindowNonOqaque(window);
              } else {
                  window.setBackground(UIManager.getColor("seaGlassTransparent"));
              }
          }
      }
docs: Tries to make the given {@link Window} non-opqaue (transparent) across
  platforms and JREs. This method is not guaranteed to succeed, and will
  fail silently if the given {@code Window} cannot be made non-opaque.
  
  <p>This method is useful, for example, when creating a HUD style window
  that is semi-transparent, and thus doesn't want the window background to
  be drawn.</p>
  
  @param window the {@code Window} to make non-opaque.