Does anyone have any idea what could be causing this strange screen artifact?

I switched GPUs from Nvidia to AMD recently, and I've mostly been very happy with it. The only thing is now I have this weird screen artifact on my main monitor. It is the exact resolution of my second monitor, which is EXTRA weird.

I'm using X11 and Openbox. I've looked at this like 7 different ways and I just can't seem to find the source of it.