That was the Note 7

The OP of the post doesn’t really confirm anything on how this even happened. But if I had to guess, it looks like the AMOLED display has “rotted” which happens when the organic materials in the display (the ‘O’ in OLED) are exposed to the atmosphere or moisture which results in decomposition of those organic materials. I don’t know the actual chemistry behind this, it’s way too out of my scope. There’s not much information I could find from a quick search regarding how this truly works.

The phone is cracked, so a gap may have formed between the display and its protective layer through which moisture could get to and cause this. But there’s also a faint black outline right around where the battery usually is, so that could’ve also done something.