Update: It also affects the desktop, I've just discovered, changing the windows desktop refresh rate from 100Hz to 60Hz simply moves the line to somewhere just above halfway ><
The fact that it moved means it's something to do with window's refresh rate I'm guessing. I could try setting it to 144Hz, but I'm not sure how??
I know an older version of Catalyst gave me an option for 144Hz desktop as I used that, but that option has disappearance.
Update: Using Custom Resolution Settings I forced 144Hz using Catalyst but the tear line was still there. NOT SURE.