I tried this on an oled display (by removing the controller and accessing row and column lines directly).

Light certainly had some effect, but I didn't get to be able to have the display show something and act as a sensor at the same time (possibly because the capacitance of the row and column lines is too high)

What specific display did you try this with? Maybe µLED displays will eventually be sensitive enough for use as 3d webcams or pupil tracking for AR/VR.

Capacitive touch screens in lenses can also track pupils using the shape of the eye.

I would love to learn more about using capacitive touch screen to track pupils. Is there a specific article or smth you can recommend?

Each pixel receives light from each point in the room, so almost all of them have the same value.

You need a pinhole to block vast majority of that light, so that each pixel on screen receives light from a different point on the other side of the pinhole.

Your OLED display doesn't have a lens though, so you would only see light differences

The keyhole of the door could serve as a pinhole.

Maybe, but why do that when you can compromise webcams and display cables?

display cables can be shielded, webcams can be covered over removed or banned from office use...the whole tempest hacking thing was intended to be applied noninvasively from a suspicious icecream truck parked outside for the last 3 days...

...if you work it for a while, you can image things in a radar like fashion. there was even a device that could be purchased that was a working television/camera, now obsolete...what you are suggesting requires physical compromise beforehand...