So this issue just came up out of the blue. No change in mods, no graphics mods to begin with, no settings changes. Running without mods doesn't make a difference.

Whenever the game's shadows shift with the time of day (every 10 seconds or so), shadows on static objects appear as thin lines, scrolling quickly upward across the object's surface. Screenshots wouldn't capture it while it happened. But imagine all the stone walls in a city with thin horizontal bands across the sides, rolling upward.

What in the world could cause something like this?