Let a host explain an absent Google button instead of hiding it

lyricanki's Linux desktop cannot use google_sign_in (no Linux
implementation) and its loopback replacement is dark until a Desktop OAuth
client exists. Hiding the button there makes the screen silently offer less
on one platform, which reads as a bug.

googleUnavailableReason renders a *disabled* button plus the reason. That is
not the "control that can never succeed" trap: the trap is a live-looking
button that reports a fake cancellation, and a null onPressed cannot be
tapped at all. Null stays the default, so every existing host is unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CFSGeWxHPY5rHUGrPqhcmG

sync_settings_ui/lib/src/sync_settings_screen.dart

@@ -163,6 +163,7 @@ class _SyncSettingsScreenState extends State<SyncSettingsScreen> {
             connected: _firebaseConnected,
             busy: _busy,
             googleAvailable: _googleAvailable,
+            googleUnavailableReason: widget.googleUnavailableReason,
             emailController: _emailController,
             passwordController: _passwordController,
             onConnectPassword: _connectFirebase,

sync_settings_ui/lib/src/sync_settings_sections.dart

@@ -10,6 +10,7 @@ class _FirebaseSection extends StatelessWidget {
     required this.connected,
     required this.busy,
     required this.googleAvailable,
+    required this.googleUnavailableReason,
     required this.emailController,
     required this.passwordController,
     required this.onConnectPassword,
@@ -20,6 +21,7 @@ class _FirebaseSection extends StatelessWidget {
   final bool connected;
   final bool busy;
   final bool googleAvailable;
+  final String? googleUnavailableReason;
   final TextEditingController emailController;
   final TextEditingController passwordController;
   final VoidCallback onConnectPassword;
@@ -54,6 +56,25 @@ class _FirebaseSection extends StatelessWidget {
             ),
           ),
           const SizedBox(height: 12),
+        ] else if (googleUnavailableReason != null) ...[
+          // Disabled, not hidden. A null onPressed cannot be tapped, so this
+          // is not the "control that can never succeed" trap -- that one
+          // looks live and reports a fake cancellation. This one says why,
+          // which beats a screen that silently offers less on one platform.
+          Align(
+            alignment: Alignment.centerLeft,
+            child: FilledButton.icon(
+              onPressed: null,
+              icon: const Icon(Icons.account_circle),
+              label: const Text('Sign in with Google'),
+            ),
+          ),
+          const SizedBox(height: 4),
+          Text(
+            googleUnavailableReason!,
+            style: Theme.of(context).textTheme.bodySmall,
+          ),
+          const SizedBox(height: 12),
         ],
         ExpansionTile(
           initiallyExpanded: !googleAvailable,

sync_settings_ui/lib/src/sync_settings_widget.dart

@@ -19,6 +19,7 @@ class SyncSettingsScreen extends StatefulWidget {
     required this.firebaseFactory,
     this.googleFirebaseFactory,
     this.googleAvailable,
+    this.googleUnavailableReason,
     this.backup,
     super.key,
   });
@@ -59,6 +60,17 @@ class SyncSettingsScreen extends StatefulWidget {
   /// unsupported even though a factory is wired.
   final bool? googleAvailable;
 
+  /// Why Google sign-in is unavailable here, shown under a *disabled* button.
+  ///
+  /// Null hides the button entirely, which stays the default. Supplying a
+  /// reason trades a silently-absent control for one that explains itself --
+  /// worth it where a platform genuinely cannot support the flow, and where
+  /// its absence would otherwise read as a bug.
+  ///
+  /// Not the same as a button that can never succeed: this one cannot be
+  /// tapped at all, so it can never report a misleading "cancelled".
+  final String? googleUnavailableReason;
+
   /// Optional local export/import section. Null omits the "Backup" section
   /// entirely (diet_guard, wake_alarm have no local backup format).
   final BackupSlot? backup;

sync_settings_ui/pubspec.yaml

@@ -5,7 +5,7 @@ description: >-
   injected closures -- no keystore or platform plugin access lives in this
   package itself.
 publish_to: 'none'
-version: 0.2.0
+version: 0.3.0
 
 environment:
   sdk: ^3.12.2

sync_settings_ui/test/sync_settings_screen_google_test.dart

@@ -141,5 +141,55 @@ void main() {
         expect(find.text('Sign in with Google'), findsNothing);
       },
     );
+
+    testWidgets('a reason renders a disabled button that explains itself', (
+      tester,
+    ) async {
+      await tester.pumpWidget(
+        _wrap(
+          SyncSettingsScreen(
+            accountLoader: () async => null,
+            accountSaver: (_) async {},
+            accountClearer: () async {},
+            sessionProbe: () async => false,
+            firebaseFactory: () async => null,
+            googleFirebaseFactory: () async => _fakeClient(),
+            googleAvailable: false,
+            googleUnavailableReason: 'Not available on this platform.',
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      expect(find.text('Sign in with Google'), findsOneWidget);
+      expect(find.text('Not available on this platform.'), findsOneWidget);
+
+      // Disabled, so it cannot report a misleading "cancelled".
+      final button = tester.widget<FilledButton>(
+        find.widgetWithText(FilledButton, 'Sign in with Google'),
+      );
+      expect(button.onPressed, isNull);
+    });
+
+    testWidgets('no reason keeps the button hidden, as before', (
+      tester,
+    ) async {
+      await tester.pumpWidget(
+        _wrap(
+          SyncSettingsScreen(
+            accountLoader: () async => null,
+            accountSaver: (_) async {},
+            accountClearer: () async {},
+            sessionProbe: () async => false,
+            firebaseFactory: () async => null,
+            googleFirebaseFactory: () async => _fakeClient(),
+            googleAvailable: false,
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      expect(find.text('Sign in with Google'), findsNothing);
+    });
   });
 }