Offline areas download an extra ring of tiles at each zoom level

lib/dev_config.dart

@@ -23,3 +23,6 @@ const int kTileFetchSecondDelayMs = 15000;
 const int kTileFetchJitter2Ms = 4000;
 const int kTileFetchThirdDelayMs = 60000;
 const int kTileFetchJitter3Ms = 5000;
+
+// User download max zoom span (user can download up to kMaxUserDownloadZoomSpan zooms above min)
+const int kMaxUserDownloadZoomSpan = 7;

lib/screens/home_screen.dart

@@ -146,7 +146,32 @@ class _DownloadAreaDialogState extends State<DownloadAreaDialog> {
   Widget build(BuildContext context) {
     final bounds = widget.controller.camera.visibleBounds;
     final maxZoom = _zoom.toInt();
+    double sliderMin;
+    double sliderMax;
+    int sliderDivisions;
+    double sliderValue;
+    // Generate slider min/max/divisions with clarity
+    if (_minZoom != null) {
+      sliderMin = _minZoom!.toDouble();
+    } else {
+      sliderMin = 12.0; //fallback
+    }
+    if (_minZoom != null) {
+      final candidateMax = _minZoom! + kMaxUserDownloadZoomSpan;
+      sliderMax = candidateMax > 19 ? 19.0 : candidateMax.toDouble();
+    } else {
+      sliderMax = 19.0; //fallback
+    }
+    if (_minZoom != null) {
+      final candidateMax = _minZoom! + kMaxUserDownloadZoomSpan;
+      int diff = (candidateMax > 19 ? 19 : candidateMax) - _minZoom!;
+      sliderDivisions = diff > 0 ? diff : 1;
+    } else {
+      sliderDivisions = 7; //fallback
+    }
+    sliderValue = _zoom.clamp(sliderMin, sliderMax);
     // We recompute estimates when the zoom slider changes
+
     return AlertDialog(
       title: Row(
         children: const [
@@ -167,12 +192,13 @@ class _DownloadAreaDialogState extends State<DownloadAreaDialog> {
                 Text('Z${_zoom.toStringAsFixed(0)}'),
               ],
             ),
+
             Slider(
-              min: 12,
-              max: 19,
-              divisions: 7,
+              min: sliderMin,
+              max: sliderMax,
+              divisions: sliderDivisions,
               label: 'Z${_zoom.toStringAsFixed(0)}',
-              value: _zoom,
+              value: sliderValue,
               onChanged: (v) {
                 setState(() => _zoom = v);
                 WidgetsBinding.instance.addPostFrameCallback((_) => _recomputeEstimates());

lib/services/offline_areas/offline_tile_utils.dart

@@ -22,12 +22,16 @@ Set<List<int>> computeTileList(LatLngBounds bounds, int zMin, int zMax) {
   }
   for (int z = zMin; z <= zMax; z++) {
     final n = pow(2, z).toInt();
-    final minTile = latLonToTile(latMin, lonMin, z);
-    final maxTile = latLonToTile(latMax, lonMax, z);
-    final minX = min(minTile[0], maxTile[0]);
-    final maxX = max(minTile[0], maxTile[0]);
-    final minY = min(minTile[1], maxTile[1]);
-    final maxY = max(minTile[1], maxTile[1]);
+    final minTileRaw = latLonToTileRaw(latMin, lonMin, z);
+    final maxTileRaw = latLonToTileRaw(latMax, lonMax, z);
+    int minX = min(minTileRaw[0].floor(), maxTileRaw[0].floor()) - 1;
+    int maxX = max(minTileRaw[0].ceil() - 1, maxTileRaw[0].ceil() - 1) + 1;
+    int minY = min(minTileRaw[1].floor(), maxTileRaw[1].floor()) - 1;
+    int maxY = max(minTileRaw[1].ceil() - 1, maxTileRaw[1].ceil() - 1) + 1;
+    minX = minX.clamp(0, n - 1);
+    maxX = maxX.clamp(0, n - 1);
+    minY = minY.clamp(0, n - 1);
+    maxY = maxY.clamp(0, n - 1);
     for (int x = minX; x <= maxX; x++) {
       for (int y = minY; y <= maxY; y++) {
         tiles.add([z, x, y]);