After downgrading my nvidia driver the issue seems to be disappeared. (I don't really understand the interdependencies of the packages but I removed the nvidia-current package and installed nvidia-glx-173; I guess my driver will not be automatically updated.)
I've been working with it for 3 hours and every window is ok; with the other driver the fourth opened maximized window was white.
After downgrading my nvidia driver the issue seems to be disappeared. (I don't really understand the interdependencies of the packages but I removed the nvidia-current package and installed nvidia-glx-173; I guess my driver will not be automatically updated.)
I've been working with it for 3 hours and every window is ok; with the other driver the fourth opened maximized window was white.