Linux troubles almost always have a solution.
But in my limited experience the troubles involving nvidia drivers are the worst.
So the problem is. I have a second monitor connected to my laptop. And sometimes, when I'm doing something that's using the graphics card - mostly Godot, that's about the only thing I've set up - the second monitor freezes. And I need to change xrandr settings and change them back to get it to work again. Doesn't matter which monitor I have Godot on.
Apparently this is just, a known bug with the NVIDIA driver... External monitor freezes when using dedicated GPU (NVIDIA Developer Forums. This person on AskUbuntu says a workaround is to enable "performance mode" in NVIDIA Settings. Sadly, um, I installed NVIDIA Settings but I don't have all the menus that the screenshot does and I can't enable "performance mode".
Missing menu options seems much more like a problem that I can solve. Just need to install the right package, or change the right setting... hopefully!