Let a game name its own host on the web sign-in screen

CHANGELOG.md

@@ -14,6 +14,10 @@ Pre-1.0, breaking changes land in a **MINOR** bump: `^0.1.0` resolves to
 [Versions and compatibility](https://eigeninteractive.com/docs/reference/compatibility)
 for how this package, the engine and the generated `eigen_api` client pair up.
 
+## [Unreleased]
+### Added
+- EngineConfig.authDomain overrides Firebase Auth's domain, so web sign-in can name your own host instead of the project's firebaseapp.com. Optional and cosmetic: unset, which is the scaffolded value, keeps the project default, and Android never shows it. It is not APP\_HOST; the value must be a Firebase Hosting domain. Applied to the generated FirebaseOptions at startup, so configure\_firebase can keep regenerating firebase\_options.dart.
+
 ## [0.4.0] - 2026-08-11
 ### Added
 - GameContentContext.transition gives a game the step into the current frame (from, to), or null when there is nothing to animate: a cold load, a rejoin, or the opening frame. Animate only when it is non-null, which replaces tracking the last rendered version in widget state. Replay supplies it too, so one animation path serves live play and replay.
@@ -162,6 +166,7 @@ server-side concern now live in the engine.
 - `google_fonts`, which fetched Inter at runtime, replaced by the bundled
 package font above.
 
+[Unreleased]: https://github.com/eigeninteractive/eigen-flutter/compare/v0.4.0...HEAD
 [0.4.0]: https://github.com/eigeninteractive/eigen-flutter/compare/v0.3.7...v0.4.0
 [0.3.7]: https://github.com/eigeninteractive/eigen-flutter/compare/v0.3.6...v0.3.7
 [0.3.6]: https://github.com/eigeninteractive/eigen-flutter/compare/v0.3.5...v0.3.6

lib/app_runner.dart

@@ -41,7 +41,16 @@ Future<void> runEngineApp({
   configureUrlStrategy();
   FlutterNativeSplash.preserve(widgetsBinding: widgetsBinding);
 
-  await Firebase.initializeApp(options: firebaseOptions);
+  // `firebase_options.dart` is generated by FlutterFire and regenerated by
+  // `configure_firebase`, so a hand-edited `authDomain` would not survive. The
+  // override is applied here instead, from ordinary deployment configuration.
+  // Unset is the normal case and leaves the generated options untouched.
+  final authDomain = config.engine.authDomain;
+  await Firebase.initializeApp(
+    options: authDomain == null
+        ? firebaseOptions
+        : firebaseOptions.copyWith(authDomain: authDomain),
+  );
   // Crash reporting and analytics are disabled in non-release builds so dev
   // events never reach production dashboards. Crashlytics has no web
   // implementation; browser errors remain visible in the console and hosting

lib/core/config/app_config.dart

@@ -41,6 +41,7 @@ class EngineConfig {
     required this.googleWebClientId,
     required this.firebaseVapidKey,
     this.appHost,
+    this.authDomain,
   });
 
   /// Origin of the EigenInteractive server, with no trailing slash and no
@@ -72,6 +73,28 @@ class EngineConfig {
   /// rather than being intercepted.
   final String? appHost;
 
+  /// Firebase Auth's own domain, overriding the project default; null keeps it.
+  ///
+  /// Purely cosmetic, and only in a browser. Web sign-in runs through Firebase's
+  /// popup, which loads `https://<authDomain>/__/auth/handler`, and Google's
+  /// account chooser names that host: by default *"Sign in to continue to
+  /// my-project.firebaseapp.com"*. Setting this replaces the string a player
+  /// reads there. Android signs in through the native Google flow and never
+  /// shows it, so an Android-only game has no reason to set this.
+  ///
+  /// **Not [appHost].** This host serves Firebase's auth handler and must be a
+  /// **Firebase Hosting** domain, whereas [appHost] is the game's own Worker.
+  /// One cannot be the other; they are sibling names on the same domain, e.g.
+  /// `auth.mygame.com` beside `mygame.com`. Setting this to a host Firebase
+  /// Hosting does not answer for breaks web sign-in outright, which is why it
+  /// stays unset until the Hosting domain exists.
+  ///
+  /// The name is only half the branding. The logo and product name on that
+  /// screen come from the OAuth consent screen in Google Cloud, which is a
+  /// separate change and needs no code. See
+  /// <https://eigeninteractive.com/docs/ship-it/deploy-the-web-app>.
+  final String? authDomain;
+
   /// Validates the deployment values before any engine service starts.
   ///
   /// [isWeb] controls the platform-specific Web Push requirement. Invalid
@@ -108,6 +131,17 @@ class EngineConfig {
       errors.add('APP_HOST must be a hostname without a scheme, port, or path');
     }
 
+    // Same shape as APP_HOST, and worth failing over rather than passing
+    // through: a malformed value reaches Firebase as the origin it builds the
+    // popup URL from, and the failure surfaces as a sign-in that never returns
+    // rather than as a configuration error.
+    final auth = authDomain;
+    if (auth != null && !_isValidHost(auth)) {
+      errors.add(
+        'AUTH_DOMAIN must be a hostname without a scheme, port, or path',
+      );
+    }
+
     if (errors.isEmpty) return;
     throw StateError(
       'Invalid EigenInteractive app configuration:\n'

test/core/config/app_config_test.dart

@@ -84,5 +84,41 @@ void main() {
         ),
       );
     });
+
+    test('treats a custom auth domain as optional', () {
+      // Every game ships without one: sign-in works on the Firebase project's
+      // own domain, and this only changes the hostname a web player reads on
+      // Google's account chooser.
+      const config = EngineConfig(
+        apiBaseUrl: 'https://game.example.com',
+        googleWebClientId: 'client.apps.googleusercontent.com',
+        firebaseVapidKey: 'public-vapid-key',
+      );
+
+      expect(() => config.validate(isWeb: true), returnsNormally);
+    });
+
+    test('rejects an auth domain given as a URL rather than a host', () {
+      // Firebase builds `https://<authDomain>/__/auth/handler` from this, so a
+      // value carrying its own scheme yields a popup URL that goes nowhere.
+      // Failing at startup beats a sign-in that silently never returns.
+      const config = EngineConfig(
+        apiBaseUrl: 'https://game.example.com',
+        googleWebClientId: 'client.apps.googleusercontent.com',
+        firebaseVapidKey: 'public-vapid-key',
+        authDomain: 'https://auth.game.example.com/',
+      );
+
+      expect(
+        () => config.validate(isWeb: true),
+        throwsA(
+          isA<StateError>().having(
+            (error) => error.message,
+            'message',
+            contains('AUTH_DOMAIN must be a hostname'),
+          ),
+        ),
+      );
+    });
   });
 }