((SunToolkit)Toolkit.getDefaultToolkit()).realSync() fails with timeout if a test with it is called under Xfce. Apparently in syncNativeQueue() in XToolkit a root window doesn't receive a necessary SelectionNotify event.
To test this, use any realSync() test: you'll see timeout. Take for instance this: closed/java/awt/Window/GetWindowsTest.
So far it's the only non-working thing in Xfce vs Gnome that I know. Also I know Xfce is not a supported platform and never will be but it's good, mature lightweight gtk-based system, so if you have a chance please take a look.