for a few days now this mark has appeared on the screen and continues to grow. what can it be?
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I don't think this is a Digitizer issue. May end up being the internal display issue spreading on pressure. You would be surprised with the cost for the fix. But have it checked with the Asus service center once. The phone is out of warranty and there's no warranty for the display issues. Check a local technician too if Asus charges you more.