[Xiaomi Mi 9 Questions & Answers] invisiblewave:
Maybe not the right sub-forum for this, but for some reason I show as not logged in on the Guides sub-forum.....

Long story short, I removed the shattered back of my phone for replacement, and in the process it appears I did something that broke the power and volume buttons. I don't know what broke, it all looks good, but I used a blade to lift the cover and in retrospect, I think heat and a suction cup would have been better.

I tried rebooting prior to installing the new rear cover, and lucky I did because I needed access to finally fix it. The buttons were completely dead, nothing worked, but if I plugged in power, it started charging, so I knew the battery and screen were still good. From adb, the device was there but unauthorized, so I was unable to boot that way (I've since rebooted and turned on USB debugging, but evidently it uses the vendor keys still when in recharge mode, and it still showed unauthorized).

None of the local shops were any help, I suspected that if I could get the boot sequence to start all would be good, and this turned out to be the case, but they were all clueless about anything other than changing the battery or the screen.

Anyway, onto the solution. The first time I got it to boot, I simply disconnected the battery, then plugged in the power using the Xiaomi charger, it wouldn't boot at all from USB power, it would only charge. As soon as I plugged it in, the power sequence started and the phone booted and ran fine. I pushed the battery connector back into place and all was good, except the problem of what happens if I power it off.

I went to developer options, checked USB debugging, and powered off, confident that even if I still couldn't see it in charging mode, I'd at least be able to manually power it on after disconnecting the battery. Well, of course, the second time it didn't work. ADB was still unauthorized, and the battery disconnection trick didn't work, it started to boot but then appeared to lose power and the screen faded. This gave me a clue that it might be possible to use the wireless charging to get it to boot, but the problem with that was that it wouldn't wirelessly charge with the battery disconnected. In the end, what worked was to loosely hold the battery connector with my finger over the contacts, while holding the phone against the wireless charger (I have the Xiaomi 65W wireless charger), in this state the battery reads 15% even though it's full, but the phone boots. I then pushed down to fully connect the battery, at which point it read 100% and was booted. It's super-fiddly to hold it all in the right position, but perseverance pays off. If you lose the connection to the battery completely at any point, the phone simply dies.

To get to the battery connection on the Mi 9 Pro, lift the wireless pad, at the top right corner of the battery you'll see a short flat black cable that runs towards the centre of the battery and then up to connect to the motherboard. Lift up on that connector with something thin to pop it off. When rebooting, hold your index finger on the connector with enough pressure to make a contact without fully reconnecting it, while holding the wireless pad against the charger. Why the hell this works I have no idea, but it does.

To operate the volume/power, just download widgets. It would be nice to have a physical volume button, but doing it from the screen works.

This was a life-saver for me because I friggin' love this phone. I have the 12/512 model, and it's so good I've finally stopped fiddling with ROMS because it just works perfectly. Unfortunately, it took me a week to figure this out, and I ordered a Mi 11 Pro in the meantime!😥

If anyone has any good ideas about an easier way for me to reboot in case it ever powers off (a simple reboot is fine), I'm all ears. I'm currently looking at having to remove the rear cover again and go through the whole rigmarole if it ever powers off.