Ungentoo

It only took him 30 minutes to add implement a button and another 30 to style it with icons

It was pain to install it cuz it's so small

I hard-bricked my Poco once again yesterday...

I would start the story once again.
When I got the Poco (Start of December) and started development work, I noticed that it worked very badly with the USB ports on the front of my PC. I continued using USB passthrough from my keyboard, but at some point I forgot about this problem and flashed the xiaomi.eu fastboot ROM to test OrangeFox, like I would normally do.
This time, flashing critical bootloader partitions (for some reason, that's what the script will flash first) failed due to bad USB connections.
Unfortunately, fastboot seems to have crashed. You can crash your fastboot if you overload it with requests, so I wasn't able to start the process again without rebooting.
Obviously, the reboot was the last thing this poor Poco was able to do.

Now, as it was in a hard bricked state (no reaction on keys,power, or USB), the only way is to flash using EDL by triggering the test points.
On Xiaomi, unfortunately, you have to use the service center account to do this or pay for shady services.
So in time, I opened my phone, and I also broke the key cable. I broke cables, but I was able to short the test points to start the EDL.

EDL flashing went successfully, even though I couldn't turn the phone on as I didn't have buttons, but the "Poco" splash appeared with battery animation.
I was sadly happy and ordered a new button PCB from Aliexpress.

(more to come...)

A week later, I resumed development of crDroid, built a very new version, and asked the community if someone could test it.
I got a reply from @Luisrosso, saying that he bricked his phone by flashing xiaomi.eu, but he used EDL test points, flashed MIUI back, and can test my crDroid version.
The very next thing he says is that my crDroid hard-bricked his phone again.

(more to come...)

A few days later, once he had trigged test points and flashed MIUI once again, he decided to try DerpFest, which hard-bricked his phone in a third time.
This was hilarious, and I didn't believe him. As it's essentially impossible to brick your phone by this, all current ROMs don't flash anything critical at all!

(more to come...)

Yesterday I received my buttons, finally turned my phone back, and the first thing I did was install the same crDroid.
It did brick my phone once again.

(more to come...)

At this point, I was very amazed, because that would mean that everything that said @Luisrosso was true, and he hard bricked his phone four times in total, three times with ROMs that I had already installed and played around with.

After the whole evening of experiments, code review, and thinking, I was able to solve this puzzle.

(more to come...)

xiaomi.eu's fastboot script flashes all images to both slots; this is a very clever move by them, so you won't end up accidentally switching slots by flashing Magisk and getting back to the old ROM you had.

The problem is that this Poco has a few bootloader images in the A/B layout, meaning you can have two different bootloaders (simplified, there are many bootloader-related partitions).

Now, every single time (6 times in total), we flashed thought EDL and didn't realize that MiFlash actually flashes only A-Slots and sets them active.
That's why we were able to restore devices as we booted into Slot A after flashing, and that's why every single ROM bricked our device.

(more to come...)

xiaomi.eu's flash script for both of us flashed garbage into bootloader partitions; hence, that's why it broke our phones.
And MiFlash flashed only A slots, leaving broken bootloader partitions in B.
Every single time we flashed custom ROMs, they unpacked themselves into B-slots and made them active.

(more to come...)