Pause tile retries when device is offline

[dougborg]

Problem

The exponential backoff for tile retries (2s → 4s → 8s → ... → 60s cap) works well for transient network errors, but when the device is fully offline (airplane mode, no wifi/cellular), retrying is pointless. It burns through the backoff progression and spams logs with DNS failures like:

ClientException with SocketException: Failed host lookup: 'tile.openstreetmap.org'
[TileLayerManager] Tile error at 14/2620/6333, scheduling retry in 2s
[TileLayerManager] Tile error at 14/2620/6333, scheduling retry in 4s
[TileLayerManager] Tile error at 14/2620/6333, scheduling retry in 8s
...

Proposed solution

Add connectivity_plus package to detect when the OS reports no network interfaces, and wire it into TileLayerManager:

1. When offline (no wifi, no cellular): cancel the retry timer and stop scheduling new retries
2. When connectivity restores: fire the reset stream immediately, reset backoff to 2s → tiles retry instantly

Limitation

connectivity_plus detects network interface availability (wifi/cellular up), not actual internet reachability. If the interface is up but DNS fails (e.g., captive portal, USB-only), the exponential backoff still handles it. This is the right trade-off — catches the common case (airplane mode, wifi off) without heavyweight reachability probes.

Implementation notes

Dependencies

• Add connectivity_plus: ^6.1.0 to pubspec.yaml
• Add ACCESS_NETWORK_STATE permission to android/app/src/main/AndroidManifest.xml (currently only has INTERNET)

Key changes to lib/widgets/map/tile_layer_manager.dart

// New fields
StreamSubscription<List<ConnectivityResult>>? _connectivitySub;
bool _isConnected = true;

// In initialize():
_connectivitySub = Connectivity().onConnectivityChanged.listen((results) {
  final connected = results.any((r) => r != ConnectivityResult.none);
  if (connected && !_isConnected) {
    _isConnected = true;
    _retryDelay = _minRetryDelay;
    _resetController.add(null); // Retry immediately
  } else if (!connected && _isConnected) {
    _isConnected = false;
    _retryTimer?.cancel(); // Stop retrying
  }
});

// In scheduleRetry():
if (!_isConnected) return; // Skip when offline

// In dispose():
_connectivitySub?.cancel();

Current state of retry system

• TileLayerManager already has exponential backoff: 2s → 4s → 8s → 16s → 32s → 60s cap
• onTileLoadSuccess() resets backoff to 2s when a tile loads
• scheduleRetry() is @visibleForTesting — connectivity tests can call it directly
• Existing test suite: 9 backoff tests in test/widgets/map/tile_layer_manager_test.dart

Existing infrastructure

• NetworkStatus service (lib/services/network_status.dart) tracks request-level status (loading/success/error) — NOT device connectivity. Don't conflate the two.
• AppState.offlineMode is a user-controlled manual toggle — separate concern
• No existing connectivity detection package in the project

Tests to add

• scheduleRetry is a no-op when offline
• Connectivity restored fires reset stream and resets backoff
• Connectivity lost cancels pending retry timer

Expected behavior after fix

Scenario	Current	After
Airplane mode on	Retries every 2s→60s forever	Stops retrying, waits for connectivity
Airplane mode off	Waits up to 60s for next retry	Retries immediately, backoff reset to 2s
Spotty wifi	Exponential backoff	Same (connectivity_plus still reports "connected")
Captive portal	Exponential backoff	Same