[Bandaid Fix] Boot looping due to power button
First and foremost, you attempt this at your own risk and I am not responsible for what happens to your device. This saved me from buying a new phone when I had an unidentifiable and persistent black screen/boot loop
Symptoms you are boot looping due to your power button sticking:
1. Black screen with intermittent vibrating
2. Phone restarts during boot animation
3. Unable to enter recovery before phone restarts
4. Unable to enter download mode before restart
5. USB JIG forces phone into download mode and restart occurs
6. Phone turns on when battery is inserted
This fix will only work if the button is STUCK and not defective. Remove back cover and battery. Remove the 6 screws on the back of the phone and gently pry the housing off.
After that, find the power button. Using a small flat head screwdriver gently push the button cover away from the mechanical button and try to press the mechanical button in to see if you can still feel it "click". After that, reassembled and test. Hope it works for you too.