Newer technologies are always exciting. Things like bezel-less displays (and their various approaches to implementing a front-facing camera), in-display fingerprint scanners, and 3D face scanning technology are just some of the things that have been introduced throughout the past 2 years and have given us a deep glimpse into the future of smartphones. But more often than not, these technologies have actually posed a challenge to the developer community, particularly to custom ROM development, as implementing these new technologies into an AOSP custom ROM isn't really a straightforward process. There are many culprits of this, including the bleeding-edge nature of these technologies (which probably aren't even supported by AOSP Android itself to begin with) as well as a lack of proper documentation.
These obstacles are eventually overcome by custom ROM developers. Recently, we got to know that OnePlus 6T custom ROMs will soon begin supporting the phone's in-display fingerprint scanner fully, bringing ROMs for this phone one big step closer to fully stable status. Now, it's the turn for the Xiaomi Mi 9, which also brings an optical in-display fingerprint scanner. The latest custom AOSP GSI by XDA Recognized Developer phhusson includes support for the Xiaomi Mi 9's fingerprint sensor, allowing users to easily unlock their device just like they would on MIUI.
The Xiaomi Mi 9's in-display fingerprint scanner on phhusson's AOSP GSI.
Other changes included in this GSI include the April security patches, miscellaneous audio fixes for some MediaTek-powered devices, allowing Samsung devices with Android Pie vendor images to boot, and more. You can check out the entire changelog here. If you want to check out this GSI on your Mi 9--or on any other Treble-compliant device, for that matter--you can download it from here, but keep in mind that being pretty much a generic, one-size-fits-all ROM, you might run into some unexpected issues with your device.