In reply to @assert-inequality:matrix.org
Observations:
Gamescope: no controller in non-steam games whether steam input is enabled or not.
Steam big picture on X: no controller with steam input enabled. Controller detected and working when steam input is disabled.
This behavior persists with gamescope regardless of running it on top of i3+picom or launching it from a new tty

{ lib
, config
, pkgs
, ...
}:
let
  userName = config.setup.userName;
  homeDir = "/home/${userName}";
  mangoDir = "${homeDir}/.config/mangohud";
in
{

  imports = [
    ./sunshine.nix
    ./gamescope
    ./mango-preset.nix
    ./steamos-utils.nix
    ./decky-loader
  ];

  config =
    lib.mkIf
      (
        config.setup.linux.gui.enable &&
        config.setup.linux.gaming.enable
      )
      {

        programs = {
          gamescope = {
            enable = true;
            capSysNice = true;
          };
          gamemode = {
            enable = true;
            settings.general.inhibit_screensaver = 0;
          };
          steam = {
            enable = true;
            protontricks.enable = true;
            gamescopeSession = {
              enable = true;
              env = {
                MANGOHUD = "1";
                MANGOHUD_CONFIGFILE = "${mangoDir}/mango.conf";
              };
            };
            remotePlay.openFirewall = true; # Open ports in the firewall for Steam Remote Play
            dedicatedServer.openFirewall = true; # Open ports in the firewall for Source Dedicated Server
            localNetworkGameTransfers.openFirewall = true; # Open ports in the firewall for Steam Local Network Game Transfers
          };
        };

        environment.systemPackages = [
          pkgs.mangohud
          (pkgs.heroic.override {
            extraPkgs = pkgs: [
              pkgs.gamescope
            ];
          })
        ];

      };
}

All the imports are irrelevant to the topic.
Side note: I even got deck loader to work!!

In reply to @assert-inequality:matrix.org
Hmm, well
I know that at least it's not gamemode because the behavior is the same. And the same workflow works flawlessly on the steamdeck (stock steamOS stable) including gyro, so it's most probably not Heroic either

In reply to @atemu12:matrix.org
Which GPU is your display hooked up to?