verified MIUI 12 EEA on Android 10 with root at 84hz working! 84hz slightly buggy and seems to slow down very rarely but it works 99% of the time, good enough for me
steps:
1. Get EEA rom at
https://piunikaweb.com/2020/08/17/x...obal-european-variants-download-links-inside/
2. Download Magisk and 84hz dtbo file (in description), and the Lanthanum apk. Then move all to USB drive.
https://xdaforums.com/Mi-9/themes/mod-overclock-screen-to-84hz-t3942063/post79776396#post79776396
3. You will wipe phone here, so you can do Nandroid backup first to be safe. Reboot to TWRP, wipe data, go back to main menu
4. Insert USB drive via type c otg, flash EEA rom, queue up magisk after it, then press start. wait til its done
5. Reboot to system, enable dev. options, enable usb debug, install lanthanum apk from your usb drive.
6. Open Lanthanum, open burger menu, open partition settings, open dtbo
7. press export image (keep this original 60hz file somewhere safe so you can flash it back later if something goes wrong)
8. Press dtbo again, flash image, find your 84hz dtbo
9. when it says done, reboot
Props to X-Dron for the original Android 10 dtbo files.
Update 1 month later with 84hz on Android 10 MIUI12 (2020 Sep 18): Figured causes for refresh rate throttling to 50-52hz. First, colder environments tend to increase throttle occurrences. Second, low screen brightness (under 20%) increases occurrences as well. During this throttling, fingerprint reader is affected because refresh rate seems to be tied to sensor polling. But no big deal. When you see this happening, just lock your phone normally, then unlock. If still at 50-52hz, then relock and unlock again until its back to 84hz. Takes 5-10 seconds max. You can soon tell when it is throttling just by eye, and you'll know when to do the lock/unlock workaround.