This issue affects both xfce4-panel and the xfdesktop4. Xfwm4 seems (I haven't checked all features) to be OK. I have a laptop with an external screen attached. The workspace spans both monitors. Because this is a laptop, I would like its screen to be my "main" monitor, that is: to display all panels and desktop icons as if it was the only monitor. This indeed happens, if the laptop screen is "left-of" or "above" the external monitor. However, when the laptop screen is "right-of" or "below", the panels and the icons are displayed on the external monitor.
There is --primary/--noprimary option in new xrandr but it doesn't seem to do anything to Xfce desktop. Perhaps Xfce could support it.
Since there's a patch in the (newer) other bugreport, let's mark this one as duplicate. *** This bug has been marked as a duplicate of bug 9338 ***