Howto: force-reboot a frozen Sony Xperia XZ1 Compact

A friend’s Sony Xperia XZ1 Compact just froze while charging: the screen stayed on, but the phone didn’t react to tapping or button presses. Holding down the Power button didn’t do anything either.

To force the phone to reboot, I pressed Volume Up and Power buttons simultaneously for about 5 seconds. Some sources say I should’ve pressed Volume Down, but that didn’t work, prompting this post.