When I ran the game on Bootcamp I had the option to play the game in Full Screen, Windowed and (if I remember correctly) Window Maximised. The last one enabled me to play on one of my monitors whilst having the other one open and available to use for other purposes. (Very helpful when making a new build as you could have the wiki and forum open on a browser.)
The Mac version is missing Window Maximised and the second monitor goes black when the game is Full Screen. This may seem like a small thing to people with only one monitor, but as twin (or triple) monitor users can probably attest to, it is bothering to not be able to use the additional monitor/s. Is there any chance that the Mac client can have Window Maximised please?