jdk version: 5.0u14 b03
tested locales: C, en, de
tested os: solaris 11, RHEL 5

Lounch SwingSet2 demo and in the Internal Frames demo, select gtk l&f. Frames have transparent edges, as seen at the attached screenshot.
Moreover, on linux, there are transparent stripes in the title bars of the internal frames (see attached screenshot). Titles are ok on solaris.