[xmonad] Xmonad sanboxed and Xinerama

[Mario Pastorelli]

Hi xmonaders,

I have decided to move my XMonad install inside a sandbox to avoid 
problems when packages are updated. I'm able to run XMonad with cabal 
exec xmonad and I'm very happy with it. My only problem is that XMonad 
cannot use Xinerama for some reason: if I configure two displays then I 
get a single display which is the union of the first one with the second 
one. I cannot find any way to use displays as independent. What I'm 
doing wrong? MATE is able to use Xinerama without problems and X11, 
XMonad and XMonad-contrib are built with xinerama:

 > ldd ../../xmonad/.cabal-sandbox/bin/xmonad
     ...
     libXinerama.so.1 => /usr/lib/x86_64-linux-gnu/libXinerama.so.1 
(0x00007f86f66e8000)
     libX11.so.6 => /usr/lib/x86_64-linux-gnu/libX11.so.6 
(0x00007f86f63b2000)
     libXrandr.so.2 => /usr/lib/x86_64-linux-gnu/libXrandr.so.2 
(0x00007f86f61a8000)
     ...

I have libxinerama:

 >  ghc -e Graphics.X11.Xinerama.compiledWithXinerama
True

And Xorg starts with Xinerama:

 > grep -i xinerama /var/log/Xorg.0.log
[    13.627] Initializing built-in extension XINERAMA

I don't know what's wrong...how can I understand what's going on?

Thanks in advance,
Mario