Backstory: I endured the same issue when my phone, an S8, mysteriously activated Universal Switch. Every time I clicked the screen, the phone, due to the gesture, registered it as me clicking the back button. Thus I couldn't do anything with the phone and got pretty stressed. Solutions: What I did to fix this was that I held down Bixby and just said "Turn off universal switch", and it worked. You can also simultaneously press the home and up volume button to trigger universal switch on and off.
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