White Border (Halo) Around Screen

A white border has appeared around the screen of my iMac. I have seen this issue reported on multiple forums and for different types of hardware. No one seems to know the cause. I seen lots of explanations from moisture behind the screen to screen resolution but none of those seem plausible. If you look at the screen you can see an even pixelation around the whole border. It seems to me to be a hardware issue because it persists during boot up. I have tried resetting NVRAM (both the keyboard boot method and the terminal method) and have reset the SMC controller but nothing has worked. When I do a screen capture (Command + Shift + 3) the screen looks perfectly fine, the halo does not appear. This seems to point to a hardware issue with the screen itself. Apparently, it is supposed to fade away completely after a while. Mine seems to be getting better over the last hour but is still present. I've included a photo of the screen taken from my iPhone.

The only thing that I think may have cause this is that my iMac got disconnected from the power source without properly shutting down.

I'm interested to know if anyone has a proper explanation for this.