I plugged my iPhone to my Mac using an USB cable and noticed the Trust this Computer dialog was randomly flashing. I couldn't tap any of the two buttons (Trust / Don't Trust) and I assumed it was because the modal was not staying on for long enough.
I tried a few more cables, I assumed they were just bad. The one I used first I only ever use for charging, so I figured its data lines were broken. The other cables didn't help, the modal was still flashing which each one. It's possible they were all bad, it was a mix of the official and 3rd party ones (I tried like five) and both kinds are known to suck.
However, trying the last cable, I noticed the modal stopped flashing and stayed on. The buttons still didn't react to tap, so I disconnected the cable figuring that it will disappear and I'll reconnect and try again with yet another cable.
However, the modal didn't dismiss itself after the cable got disconnected. Instead, it stayed on, unresponsive. I can't use the area outside of the modal to do anything, I can't even slide up to unlock (this is an iPhone X) and FaceID failed too many times and now I need to enter my unlock code, which I can't do, because the modal is over the numpad.
I wanted to restart the phone, but iPhone X reset is volume up, volume down, power and then slide on the screen. I can't use the shutdown slider because the fricking modal is over it, too!
Is my only hope now to let the phone discharge enough so that it dies and recharging it to restart it to make the modal go away? And if so, can I make the phone discharge faster? I can't even turn on the torch and tapping the screen to keep it awake and using energy gets really old really fast. I always hated these undismissable modals, but I never expected it was mutual.
Best Answer
I found out that the slider is not necessary. Pressing and releasing volume up, the pressing and releasing volume down and the long-pressing the power button turned the phone off.