I honestly have no clue
Ok, so I quickly tested:
- My FP5 on a custom A15 does indeed abruptly shutdown when holding volume down for 16s
- When I plug it in a charger after this, it seems to start powering back on it’s own (I have the warning splash screen saying it will boot a custom OS), but this then seems to fail (short vibrate and white horizontal line on a random place on the screen) and it falls back to the powered-off charging screen. I think this is the feature allowing the phone to start without the power button, but there is a bug and it fails.
- If while normally off I hold volume down for a 15s, it enters the recovery menu, which should theoretically allow to boot, but since I can’t confirm any options on mine without the power button, I can’t boot.
- If I hold volume down button when it’s in recovery mode or on the custom OS splash warning, it as well abruptly shuts down and in some cases refuses to charge nor react to other volume down long presses. Sometimes the power button works to start it again, but sometimes I’ve had to re-insert the battery to be able to get anything going.
I’ll stop investigating for now, I’m sure there is some specialised android dev article that talks about those things somewhere (didn’t find any in a few minutes search).
Note: maybe do a backup of important data if you want to keep messing around like I did
EDIT : I could have just read the latest post in the conversation above: Software update: FP5 A.14 MR4/FP5.UT2H.B.088.20241122 - #26 by L1z4rD