This is Debian bug http://bugs.debian.org/254523 Not sure if this is xfwm4's fault or the panel's fault but either way when you maximise a window and your panel is set to be on top (which is the default) it gets maximised behind the panel. It doesn't get maximised behind the taskbar. I'm not sure if this relates to the discussion on the -dev list about partial struts etc. Let me know if this is going to be fixed in the 4.0.x branch. aThanks.
assuming i'm understanding you correctly, for 4.0.x this is normal behavior. to avoid this, the user needs to use workspace margins. i don't think partial struts are going to be backported to 4.0.x.
This is not a bug, as the panel doesn't set/use struts (in 4.0 branch). That's why we added the margins to xfwm4, so please use the margins.