Visible frame not becoming Transparent in Ubuntu

I have a frame with 2 buttons and an undecorated frame. When I click on the first button on the first frame ,the undecorated frame remains opaque and when I click on the 2nd button the undecorated frame becomes transparent. But on ubuntu the undecorated frame does not become transparent from the second click onwards.

Steps to reproduce:

1.Run the attached testcase (TestOpacity.java)
2.Click the 2nd button (Not Opaque)on the first frame.
  Notice the undecorated frame becomes transparent.
3. Now click on the 1st button (Opaque) . The undecorarted frame becomes opaque.
4. Now again click on the 2nd button.
If you see that the frame is not becoming transparent the issue is reproduced.
This works fine on Windows.

I reproduced this on Ubuntu with Jdk7b60 pit build.