{
  config,
  lib,
  pkgs,
  util,
  ...
}:
# Waybar: a top status bar reading system state at a glance.
let
  waybarModulesFor =
    position:
    map (module: module.name) (
      lib.sort (a: b: a.order < b.order) (
        lib.filter (module: module.position == position) registeredWaybarModules
      )
    );

  registeredWaybarModules = lib.mapAttrsToList (
    name: module:
    module
    // {
      inherit name;
    }
  ) hostWaybarModules;

  hostWaybarModules = lib.attrByPath [ config.networking.hostName ] { } config.registry.waybar;

  registeredWaybarModuleSettings = lib.mapAttrs (_name: module: module.settings) hostWaybarModules;

in
{
  options.modules.desktop.session.waybar.enable = lib.mkEnableOption "the Waybar status bar";

  config = lib.mkIf config.modules.desktop.session.waybar.enable {
    # A symbols font backs the module glyphs, so pango falls back to it for the
    # codepoints Stylix's monospace font does not carry.
    fonts.packages = [ pkgs.nerd-fonts.symbols-only ];

    home-manager.users.${config.user.name}.programs.waybar = {
      enable = true;

      # A user service bound to graphical-session.target, which uwsm activates,
      # so the bar comes up with the session and no compositor exec-once.
      systemd.enable = true;

      settings.mainBar = lib.mkMerge [
        {
          layer = "top";
          position = "top";
          height = 30;
          spacing = 8;
          margin-right = 8;

          modules-left = waybarModulesFor "left";
          modules-center = waybarModulesFor "center";
          modules-right = waybarModulesFor "right";
        }
        registeredWaybarModuleSettings
      ];
    };

    registry.waybar.${config.networking.hostName} = {
      clock = {
        position = "center";
        order = 10;
        settings = {
          format = "{:%a %d %b  %H:%M}";
          tooltip-format = "<tt>{calendar}</tt>";
          calendar = {
            mode = "month";
            # Underline the current day so it stands out in the grid.
            format.today = "<b><u>{}</u></b>";
          };
        };
      };

      battery = {
        order = 70;
        settings = {
          format = "{icon} {capacity}%";
          format-charging = "${util.glyph "f0e7"} {capacity}%";
          format-icons = [
            (util.glyph "f244")
            (util.glyph "f243")
            (util.glyph "f242")
            (util.glyph "f241")
            (util.glyph "f240")
          ];
          states = {
            warning = 20;
            critical = 10;
          };
        };
      };
    };
  };
}