Screen shot: https://imgur.com/a/fjOYlMS
You cannot move in the improperly rendered area. You need to click on the left or right edges of the screen to be able to move.
I attempted to run with lower graphics settings, lower resolution, and a different character. Always the same missing ground. Changing graphics settings does cause the mini-map contents to disappear, until you restart the game. https://imgur.com/z5SPAGN
The game files failed verification. Turns out my Steam filesystem is corrupt. This is most likely the cause of the problem. Steam is refusing to install on the new fs and it’s in a pool with other filesystems I need, so may take a while to verify. I’ll report back, and accept Llama8’s comment as the solution as soon as I can.