[Brandon]

I haven't found a way to do it so I'm submitting it as a feature request, when I alt tab the game pause which is a bit annoying when playing turn based multiplayer matches because while I like to see what my friends are doing sometimes I have to alt tab to use the browser, chat etc... so it would be  nice to have an option to disable the pausing while the game is not focused

[pAter]

I second this.  It causes Sync issues as well!
Perhaps a "Borderless Window" would be ideal?

On that note, when I play in window mode at 1920x1080 the window actually spills into my other monitors.

But the pausing needs to be addressed.

[SAM]

In windowed mode, if you minimise down / up (continue to run the game in the task bar) you can use other open applications like Steam chat without putting the game into pause.