This is 2014, the lack of this functionality is kind of embarrassing. At least the game itself is awesome enough that I'll keep playing without it, but it would be REALLY nice to have this.

I think it is borderless windowed even if the option is labeled Fullscreen. I can alt tab freely without a black screen and my second monitor doesn't freeze when I'm on Endless Legend. My cursor can also leave the window freely. This makes me think that the more classic Fullscreen is what's missing.

Fullscreen seems to behaves as "Borderless Window" for me. Make sure that the game and your desktop are using the same resolution, this might be it.