Okay, thanks for the feedback !
The inappropriate window position is just because for some reasons Linux count 0@0 with the decorations whereas windows count it without the decorations so I should probably make them appear at 20@20 or something like that.
By the way the new SDL version (2.0.4) offers some new nice options to handle the windows but unfortunately it is still in development. But if you download it and compile it you'll see that the windows won't have decorations anymore and you'll be able to drag them from their title bar, exactly like inside the world.