Fluxbox does not currently support partial struts. It would be nice, but it's difficult to implement. A middle ground would be creating a full strut for the window. According to the EWMH spec, the client application may set both in order to accommodate window managers that do not support partial struts. However, we should do that too, at least until we support partial struts.

It's worth noting that _NET_WM_STRUT_PARTIAL does not appear in our _NET_SUPPORTED, so xfce4-panel can theoretically tell this is the case and change its hints accordingly. You should probably report this to them as well.