In my case, windowed mode does not seem to work.
My desktop's resolution is set at 1920 x 1200 : it is the native resolution of my 25½-inch monitor screen.
In fullscreen mode, I experience no problem whatsoever, and I can easily alt-tab to minimize the game if I want to surf in the web browser : alt-tabbing back into the game is then accomplished very quickly.
When I try the new windowed mode, offered on Steam, the "window" covers the whole screen, with no window borders, at a 1920 x 1200 resolution which makes that mode apparently no different from the fullscreen mode. But there is one annoying difference :
In windowed mode, alt-tab minimization does not work : the game's screen does not move, it does not collapse to the desktop's taskbar.
Consequently, I only use the fullscreen mode. But I would like to create a window that would be a little smaller than my desktop's screen, with mouse-accessible, normal window borders.
Why is my windowed mode apparently not a true "window", appearing as a static full screen with no borders and no alt-tab functionality ?