Brandon Allbery
I understand the original message as saying that it recognized screens correctly before xmonad was moved to a sandbox. This makes me wonder if the tests are being done with a different xmonad library and/or the sandbox is somehow not seeing a Xinerama-enabled X11 (this can happen if the original was installed globally via OS package but the sandboxed build is using a local X11 bindings package, and the Xinerama developer headers/libraries weren't installed, for example).
This is precisely what happened to me when I moved from my package manager's version of xmonad to building in a cabal sandbox. You need to have a few of the -dev packages installed so that the Haskell X11 package can build with Xinerama support. -- Peter Jones, Founder, Devalot.com Defending the honor of good code