I use arch linux with the current xmonad release and an xrandr dual
monitor setup. Everything works fine but no qt context menus show up on
the second screen. I found several articles reporting the same problem
The bug report cites _NET_WORKAREA, which is not set by released versions of xmonad; Qt may be doing something silly like using the virtual root window extent in that case. We have a patch in darcs that adds _NET_WORKAREA support but I don't know if it supports multiple screens.