different behaviour of JPopup menu on windows and linux

[CharlesRiver]

Hi, there, I met a wierd problem. I wrote a inherited JPanel and
implemented a JPopup menu when right mouse button pressed. The mother
Jpanel, from which it is inherited, also implemented a Jpopup menu when
right mouse pressed. I deployed the application as JNLP, but found on
windows computers, the JPopup menu on parent panel is not masked, so
imagine the mess. While on linux, the popup menu from the parent
JPanel is totally masked. (as expected)