Screen has white vignetting border halo - MacBook Pro, Retina, 15 inch, Mid 2015, Mojave 10.14.6

All of a sudden after not being able to get my screen to wake from sleep and having to hard reset by holding the power button my screen came back on with a white vignetting border halo as others have posted about many times (see https://discussions.apple.com/thread/7960425).

I tried resetting the SMC (https://support.apple.com/en-us/HT201295) and PRAM/NVRAM (https://support.apple.com/en-us/HT204063) and even ran a full system check with TechTool Pro 12 but everything passes. The suggestion from the referenced thread to change the resolution and desktop background image also failed to fix it so I turned my machine off for 30 min and checked again. It noticably faded.

I left it off overnight and it's now gone. What is this all about?