Either games spawning on the wrong monitor and not reacting well to you moving the fullscreen window to the other monitor, or mouse input issues. Latest I've had was L.A. Noire, which locks the mouse to a portion of the screen and doesn't allow you to freely turn the camera. (I just tested it again and now it seems to work fine though! I hope that persists.) Quake II doesn't allow you to move the mouse at all, or rather only in what seems in like a 2 pixel wide boundary in the middle of the screen. No such issues if the other monitor is turned off or configured to be on the right side. I've encountered more games that had issues with this in the past but these two are the recent ones I've had trouble with since setting it up like this again.

Yeah. Flakes are essentially three things (or four if you count the new CLI):

1. Lock files for inputs (like NPM)
2. A defined output layout (so, every flake has its packages at packages. for example)
3. Pure mode (don't worry about it unless you read from arbitrary locations in the file system or try to download files without a hash)

That's it, essentially nothing else changes. It's just a different entry point to Nix code including NixOS configurations.

Here's a great article (apparently, I have only skimmed it myself) explaining flakes more in detail: https://jade.fyi/blog/flakes-arent-real/

Flakes on the system level aren't too bad. You can pretty much just keep your configuration.nix, but now you call that from a flake.nix. The difference is you remove all your nix-channels and you specify your nixpkgs in your flake.nix. So its really using a flake instead of nix-channels.

The cool part is when you nixos-rebuild the first time, it will save your nixpkgs version in a flake.lock. Then it will stay that way until you choose to upgrade with nix flake update. Nice and stable.

I felt the same, but I'm reading through this book and so far it's been helpful for understanding and setting up system flakes: https://nixos-and-flakes.thiscute.world/

use flakes

NixOS can boot from a file system that only has /nix, since essentially the kernel command line has init=/nix/store/.../init. Everything else will be created during boot by that if it isn't already there. So technically you could only mount /nix and you would get a blank system every time you boot (but that wouldn't be very useful in most cases). Mounting these is done in the initrd.

A lot of people have a setup where only select files are mounted from a persistent partition, such as /var/lib/postgresql, basically anything they want to keep across reboots, so that the rest is discarded when they reboot. This prevents the system from accumulating junk over time, from services you once used to have but no longer have running, and so on. Personally I found it too much of a hassle to keep track of what files I want to keep, so I save the entire /etc and /var. I still keep the tmpfs though because it's pretty cool.