I am running minecraft 1.12.2. I had some other mods installed but have removed everything except better portals and forgelin and the issue remains.
When I make a nether portal, the screen splits into 4 parts, with 1 quarter showing the normal view, and another quarter showing what I imagine is a render of the other world (e.g., the nether if you are in the overworld, or the overworld if you are in the nether). It is very weird.
Any ideas on how to resolve this? Playing on an Acer predator 2019 with Windows 10.

Screenshot below is having just entered the nether, and looking back towards the portal.

Duplicate of #541