{ lib, pkgs, ... }:

let
  # GNOME/mutter!3797, pinned because the MR is still under development.
  mutterTearingRevision = "a8d56d423edde9e6d76f6b9cab663d82c65cf1a2";
  mutterTearingPatch = pkgs.fetchurl {
    url = "https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3797.diff";
    hash = "sha256-3up5PsHX+BSQRKb9JS5bnJLPaTGmvV55M18tKkepIng=";
  };
in
{
  nixpkgs.overlays = [
    (final: prev: {
      mutter = prev.mutter.overrideAttrs (old: {
        pname = "mutter-tearing";

        # nixpkgs leaves Mutter's verbose topics compiled out by default.
        # This is required for MUTTER_DEBUG=kms to emit the async commit flags.
        mesonFlags = (old.mesonFlags or [ ]) ++ [ ];

        # The MR currently targets Mutter 51. Apply it to the packaged Mutter
        # 50 source, then resolve the four small context conflicts separately.
        prePatch = ''
          ${old.prePatch or ""}

          set +e
          patch --batch -p1 --forward < ${mutterTearingPatch}
          mr_status=$?
          set -e

          if [ "$mr_status" -ne 0 ]; then
            reject_count="$(find . -name '*.rej' -type f | wc -l)"
            if [ "$reject_count" -ne 3 ]; then
              echo "Unexpected number of Mutter !3797 rejects: $reject_count" >&2
              exit 1
            fi
          fi

          patch --batch -p1 --forward < ${./mutter-50-tearing-compat.patch}
          find . -name '*.rej' -type f -delete
        '';

        passthru = (old.passthru or { }) // {
          tearingMr = 3797;
          tearingRevision = mutterTearingRevision;
        };
      });
    })
  ];

  environment.sessionVariables = {
    # Mutter keeps tearing support behind an experimental runtime switch.
    MUTTER_DEBUG_EXPERIMENTAL_FEATURES = lib.mkDefault "tearing";

    # Temporarily expose whether async atomic commits reach KMS or fall back
    # to synchronized presentation. Remove this after collecting the log.
    # MUTTER_DEBUG = lib.mkDefault "kms";
  };
}