I solved it checking if height is correct and then, adapting my resize system to the height-titlebar_size:
sys.width = 1280;
sys.height = 800;
sys.bpp = 32;
sys.titlebar = 0; // titlebar size
* * *
sf::Window proof;
proof.Create(sf::VideoMode(sys.width, sys.height, sys.bpp), "");
sys.titlebar = sys.height - proof.GetHeight();
proof.Close();
if (sys.titlebar != 0){
sys.height -= sys.titlebar;
}
* * *
App.Create(sf::VideoMode(sys.width, sys.height+sys.titlebar, sys.bpp), title);
Thanks for the help, Laurent!