When turning off Application Frame in InDesign there's a small dark horizontal bar in the upper left corner of my screen

I have seen this too but I agree it wasn’t distracting and I ignored it mostly. I think it could be related to the screen recognition issue where windows does not fit to screen initially.

Yes, I have the same thing for a few months already. Since it doesn’t disrupt any functionality, I’m too lazy to do the standard troubleshooting steps of resetting preferences and, more importantly, caches that I’d recommend to anybody in this case:

https://www.rockymountaintraining.com/adobe-indesign-rebuilding-preferences-cache/