Xiaomi's co-founder, Lin Bin, has recently uploaded a video to Weibo that reveals an improved version of the in-display fingerprint reading technology already in use on commercial devices. The most obvious change is the larger active authentication area, which now measures 25 x 50.2 mm. This solution will probably be used in the Xiaomi Mi 9 flagship.
https://www.notebookcheck.net/The-Xiaomi-Mi-9-flagship-will-come-with-improved-in-display-fingerprint-reading.395096.0.html