nixos/modules/system/core/boot.nix

{...}: {
  flake.nixosModules.bootloader = {
    pkgs,
    lib,
    ...
  }: {
    boot = {
      loader = {
        timeout = 2;
        efi = {
          canTouchEfiVariables = true;
        };
        # Pinned explicitly. grub.enable defaults to true, so this host silently
        # switched from systemd-boot (used at install time) to GRUB when this
        # block was added. NixOS never removes the previous bootloader's files
        # or its NVRAM entry, so both lived on the ESP and the firmware kept
        # booting the frozen systemd-boot menu. Keep this explicit, and never
        # set systemd-boot.enable alongside it.
        #
        # Note what this does and does not do. Both of these lines are no-ops
        # against nixpkgs' defaults (grub.enable is already true,
        # systemd-boot.enable already false); their value is that a conflicting
        # definition elsewhere now fails loudly instead of silently winning. They
        # delete nothing — stale EFI/systemd/, loader/ and the leftover NVRAM
        # entry have to be removed by hand on any host installed with
        # systemd-boot. Done on shadow; NOT yet verified on t480.
        #
        # This is stock UEFI GRUB: NixOS installs EFI/<distro><esp>/grubx64.efi
        # (here EFI/NixOS-boot/grubx64.efi) and registers a matching NVRAM entry
        # via efibootmgr, because canTouchEfiVariables is true. That NVRAM entry
        # is what makes GRUB the default; NixOS does not manage the removable
        # fallback path EFI/BOOT/BOOTX64.EFI and we deliberately do not ask it to
        # (grub.efiInstallAsRemovable, which would need canTouchEfiVariables=false,
        # is only for firmware that ignores NVRAM entries — neither host needs it).
        #
        # On shadow that fallback path is NOT empty, though: bcdboot left a copy of
        # the *Windows* boot manager at EFI/BOOT/bootx64.efi (byte-identical to
        # EFI/Microsoft/Boot/bootmgfw.efi). Normal boots are unaffected because the
        # firmware follows its NVRAM entry to GRUB. But it does mean that if NVRAM
        # is ever cleared — CMOS reset, battery, firmware update — the fallback
        # lands on Windows rather than GRUB. Left as-is by choice; recover by
        # rerunning `nixos-rebuild boot` to reinstate the NVRAM entry.
        systemd-boot.enable = false;
        grub = {
          enable = true;
          efiSupport = true;
          device = "nodev";
          theme = pkgs.catppuccin-grub;
          # Bound the menu so the ESP cannot fill up with old kernels.
          # install-grub.pl unlinks any $boot/kernels/* not referenced by a kept
          # generation, so this genuinely caps ESP usage rather than just
          # trimming the menu.
          #
          # Sizing, measured on shadow rather than assumed. Generations share
          # aggressively in practice: 10 kept generations referenced 1 bzImage
          # (13.3MiB) and 3 initrds (~33.4MiB each) for 114MiB total, i.e. ~11MiB
          # per generation, not ~47MiB. Only a kernel or initrd change mints a new
          # file — and the initrd does change more often than you'd expect, since
          # nixpkgs embeds /etc/modprobe.d/nixos.conf inside it, so every kernel
          # module parameter edit produces a fresh one.
          #
          # So the real bound is between ~11MiB and ~47MiB per generation
          # depending on churn: 10 generations costs ~115MiB typically and ~470MiB
          # worst case, against a 1GiB ESP that also carries grub/x86_64-efi, the
          # catppuccin theme, and Windows' own boot files (bootmgfw.efi + BCD,
          # ~30-60MiB) since Windows was bcdboot'd onto this same ESP. 10 keeps
          # the worst case under half the partition.
          #
          # Keep this conservative: overflow fails as `copy ... or die` in
          # install-grub.pl, which runs *after* the system profile has already
          # been switched, so the result is an activated but unbootable
          # generation. t480 reaches this module too and its ESP may be smaller.
          configurationLimit = 10;
        };
      };
      plymouth = {
        enable = true;
        theme = "catppuccin-mocha";
        themePackages = with pkgs; [
          (catppuccin-plymouth.override {
            variant = "mocha"; 
          })
        ];
      };
      kernelPackages = pkgs.linuxPackages_latest;
    };
  };
}