With xorg 6.8 and compositor enabled. XFCE 4.2 Beta 2.

When the taskbar plugin is enabled and expand to fill screen option is enabled,
the panel shows incorrectly when a new application with systray icon is open.
Others plugins like clock dissapear. Is necessary to hide and show again the
panel to view correctly. 

And sometimes, another strange effect is that the panel increese his transset
value when change the fixed size of the taskbar plugin.

Regards.