fix: convert pinch scale to zoom through the CRS instead of log2

lib/src/gestures/map_interactive_viewer.dart

@@ -1392,10 +1392,12 @@ class MapInteractiveViewerState extends State<MapInteractiveViewer>
 
   // Utilities
 
+  // Through the CRS rather than `log2`: the ratio between zoom levels is not
+  // 2 in every CRS (e.g. `Proj4Crs` with custom resolutions).
   double _getZoomForScale(double startZoom, double scale) {
-    final resultZoom =
-        scale == 1.0 ? startZoom : startZoom + math.log(scale) / math.ln2;
-    return _camera.clampZoom(resultZoom);
+    if (scale == 1.0) return _camera.clampZoom(startZoom);
+    final crs = _camera.crs;
+    return _camera.clampZoom(crs.zoom(crs.scale(startZoom) * scale));
   }
 
   Offset _rotateOffset(Offset offset) {

test/gestures/pinch_zoom_crs_test.dart

@@ -0,0 +1,97 @@
+import 'package:flutter/widgets.dart';
+import 'package:flutter_map/flutter_map.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:latlong2/latlong.dart';
+import 'package:proj4dart/proj4dart.dart' as proj4;
+
+// Resolutions with a ratio other than 2 between levels (the Swiss LV95 grid):
+// 2.5 -> 2 -> 1.5 -> 1 m/px from zoom 22.
+const _resolutions = <double>[
+  4000, 3750, 3500, 3250, 3000, 2750, 2500, 2250, 2000, 1750, //
+  1500, 1250, 1000, 750, 650, 500, 250, 100, 50, 20, //
+  10, 5, 2.5, 2, 1.5, 1, 0.5, 0.25, 0.1,
+];
+
+final _customCrs = Proj4Crs.fromFactory(
+  code: 'TEST:CUSTOM',
+  proj4Projection: proj4.Projection.get('EPSG:3857')!,
+  resolutions: _resolutions,
+);
+
+/// Two fingers around [c], [scale] times their initial 100 px apart.
+List<Offset> _fingers(Offset c, double scale) {
+  final v = Offset(50 * scale, 0);
+  return [c - v, c + v];
+}
+
+Future<MapController> _pumpMap(
+  WidgetTester tester, {
+  required Crs crs,
+  required double zoom,
+}) async {
+  final controller = MapController();
+  await tester.pumpWidget(
+    Directionality(
+      textDirection: TextDirection.ltr,
+      child: Center(
+        child: SizedBox(
+          width: 400,
+          height: 400,
+          child: FlutterMap(
+            mapController: controller,
+            options: MapOptions(
+              crs: crs,
+              initialCenter: const LatLng(46.5, 6.6),
+              initialZoom: zoom,
+              minZoom: 8,
+              maxZoom: 28,
+            ),
+            children: const [],
+          ),
+        ),
+      ),
+    ),
+  );
+  return controller;
+}
+
+/// Spreads two fingers through [scales].
+Future<void> _pinch(WidgetTester tester, List<double> scales) async {
+  final c = tester.getCenter(find.byType(FlutterMap));
+  final start = _fingers(c, 1);
+  final g1 = await tester.startGesture(start[0], pointer: 1);
+  final g2 = await tester.startGesture(start[1], pointer: 2);
+  await tester.pump();
+  for (final scale in scales) {
+    final p = _fingers(c, scale);
+    await g1.moveTo(p[0]);
+    await g2.moveTo(p[1]);
+    await tester.pump();
+  }
+  await g1.up();
+  await g2.up();
+  await tester.pumpAndSettle();
+  // Let the double tap timer expire.
+  await tester.pump(const Duration(seconds: 1));
+}
+
+void main() {
+  group('pinch zoom', () {
+    testWidgets('doubles the scale when the ratio between levels is not 2',
+        (tester) async {
+      final controller = await _pumpMap(tester, crs: _customCrs, zoom: 22);
+      await _pinch(tester, [1.5, 2]);
+      expect(
+        _customCrs.scale(controller.camera.zoom) / _customCrs.scale(22),
+        closeTo(2, 1e-3),
+      );
+    });
+
+    testWidgets('adds one level in EPSG:3857', (tester) async {
+      final controller =
+          await _pumpMap(tester, crs: const Epsg3857(), zoom: 10);
+      await _pinch(tester, [1.5, 2]);
+      expect(controller.camera.zoom, closeTo(11, 1e-3));
+    });
+  });
+}