Whatever kind of polarizers the Navi screen uses internally to filter light and compose the image, the final outcome is horizontally polarized light and thus blocked by polarized shades.
This does not happen with other screens: it does not happen with my TV set nor with my computer, so obviously there are different technologies used in screen design and manufacture.
My question is whether there might exist a replacement screen for the navi that would use a different technology.
Thanks everyone for your answers.