'Window' acts as 'Full screen'?
 

It seems, that although marking the game as 'window', if the resolution of the window is equal or greater then the resolution of the screen, the game acts as if 'Full screen' (no background play)

Is there a solution for this bug?

Temujin

That's not a bug, it's intentional. I don't think many would want to play a game they couldn't see all of.

If you want to get it in a window that is larger than your physical screen, then I guess the only thing you could try would be to activate any 'virtual desktop' option in your graphics card, that allows your whole desktop to be a higher res than your viewing window. That should let you run it windowed at a res larger than your physical screen.