Not sure if this is related to your issue, but drop down menus in Java seem to have issues with some Linux desktop "themes" (GTK themes, I think). For example, on CentOS the default theme causes the menus in Netbeans to be drawn without any borders. Changing the theme to something else fixes the problem.
I have had this issue for the last couple of releases and I can't narrow down the issue. It works fine on Windows.