This would be absolutely amazing if it turns out to be true. Nikon have filed a couple of patents for hybrid viewfinders in the past, but eagle eyed Flickr user, Anankhepi believes it may be on the way. Two dark circles above the viewfinder in the leaked Nikon D850 photos are the clue.
On first glance, one might simply believe them to be screw holes, but they don’t exist in any other camera in Nikon’s lineup. And never have. He believes these holes contain sensors to detect when the camera is being held up to your eye. This video from Angry Photographer goes into more detail about how this may work.