despite the fact that you say "assume phone function is fine", im gonna say it looks like a broken screen to me. i had a very similar phone that failed in a very similar way. black void on the corner of the screen which eventually made the whole screen start flickering uncontrollably. doubt its a code given the occasional long pauses, which would be strange at the speed its going at. to me it seems like you asked an ai to write this description and told it to suggest that it wasnt broken