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).

I don't think we have enough information to diagnose this.
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