A diagram from a Xiaomi patent has surfaced online and it shows a device with an in-display selfie camera. The patent which was applied for back in November 2018 shows there is a camera behind the screen as well as a light sensor. A rough translation of the patent description says there is a secondary display under the main display.
The light sensor works with both screens in a way that the camera can become visible at required times or invisible when not in use. It is a bit hard to explain since the description isn’t in English.