When drawing in 3D, you tell your 'camera' that's viewing a scene to have a close end cutoff and a far end cutoff for objects so you don't put stuff that wouldn't be useful to see. Since extra zoom is hacked in, you can see where they put the far cutoff distance. Chats and vends are 'painted' on top of the 3D picture, so they aren't part of the 3D objects being drawn.
So it's not a black thing coming/rising off the corners, it's the 3D objects sinking below the far point of view that's programmed in the client.
