You could always use EM shielding. Faraday cages are a thing and are used for this purpose in real-world high security installations as I understand it. But i

You could always use EM shielding. Faraday cages are a thing and are used for this purpose in real-world high security installations as I understand it. But if the attacker can see in a line-of-sight way the same screen the intended recipient can, yes, there's really no defence.