Trying the tip of turning high contrast on/off worked for me! Thanks
@lumianok smart! For good measure I also tried toggling between high contrast black and high contrast white, don't know if that really made a difference.
I was having the same issue of thick borders on 3 sides, and the weirdo old max/min/close button controls at top right. Not just in File Explorer either. Edge also had the borders problem. And the old controls were showing up in Skype MSI also (though not the Skype UWP). Image shows Slack at top working fine, File Explorer on right, and Edge on left, before the fix. Let's hope the fix sticks and I don't have to keep going back to toggle high contrast mode.
BTW I didn't have Dark Mode turned on, was happening evening without that (though I wasn't have the light/dark patch areas in Explorer like OP)
