Hmm, that's really odd. Unfortunately it's either something with that version of osx or some software for extending the desktop, or an issue with the unity 3d engine in how it relates to those other components. It's all below the level of what we can mess with.
The one thing to check is to be sure that you put it in windowed mode via the in-game settings. If you did it via hotkeys, that can definitely cause strangeness on some OSes.
I don't recall whether I've ever tried to to run TLF on the other monitor, so its possible this has never worked.
OS/Mac's dual monitor support is pretty weird when it come to getting some applications to play nicely. What I noticed further was that I've no way to get the game to work fullscreen on the second monitor either. But yeah this is more likely an issue of the OS's window/device handler not actually tracking correctly or the library (in this case Unity not communicating back at an interrupt level).
No time for me to be curious (so much work to complete today) and see if I can trap the issue, I just thought I'd see if there was a known work around, so I can continue working and have the game running on the second monitor to play in short bursts to see what's changed.
Thanks for answering so quick.
