A patent filed by Xiaomi for a full-screen smartphone utilizing an under-display camera has been unearthed. The patent application was made with the State Intellectual Property Office of China in November 2018, and it briefly describes Xiaomi's solution for the feature, which appears to involve the smartphone using two screens.
https://www.notebookcheck.net/Xiaomi-could-beat-Samsung-in-producing-a-full-screen-smartphone-with-an-under-display-camera.422924.0.html