SDL2 changed the way it handles keyboard input on linux and now in fullscreen ALT + TAB is fully working on linux. I don't know the details, but it would be cool if SFML could handle it in the future. Most of 2D games are windowed, but some of them are really good on fullscreen and I think it would be a good feature to implement in 2.1. (not to mention when you use sfml for windowing and opengl for 3d rendering!)