diff --git a/lib/widgets/map_svg_color_mapper.dart b/lib/widgets/map_svg_color_mapper.dart
index 50dda7e6..bf3f2405 100644
--- a/lib/widgets/map_svg_color_mapper.dart
+++ b/lib/widgets/map_svg_color_mapper.dart
@@ -1,3 +1,4 @@
+import 'package:flutter/foundation.dart' show mapEquals;
 import 'package:flutter/widgets.dart';
 import 'package:flutter_svg/flutter_svg.dart';
 import 'package:icarus/providers/user_preferences_provider.dart';
@@ -12,24 +13,26 @@ class MapSvgSourceColors {
 }
 
 class MapSvgColorMapper extends ColorMapper {
-  const MapSvgColorMapper(this.replacements);
+  MapSvgColorMapper(Map<int, Color> replacements)
+      : replacements = Map.unmodifiable(replacements);
 
   MapSvgColorMapper.forPalette(MapThemePalette palette)
-      : replacements = {
+      : replacements = Map.unmodifiable({
           MapSvgSourceColors.base.toARGB32(): palette.baseColor,
           MapSvgSourceColors.detail.toARGB32(): palette.detailColor,
           MapSvgSourceColors.highlight.toARGB32(): palette.highlightColor,
-        };
+        });
 
   /// The map in [palette] as cones reveal it: the same art with its floor
   /// in [revealedFloorColor] (see `lib/view_cone/cone_coverage.dart`).
   MapSvgColorMapper.revealed(MapThemePalette palette)
-      : replacements = {
+      : replacements = Map.unmodifiable({
           MapSvgSourceColors.base.toARGB32(): revealedFloorColor(palette),
           MapSvgSourceColors.detail.toARGB32(): palette.detailColor,
           MapSvgSourceColors.highlight.toARGB32(): palette.highlightColor,
-        };
+        });
 
+  /// Unmodifiable: mappers are compared by these, so they never change.
   final Map<int, Color> replacements;
 
   @override
@@ -48,4 +51,18 @@ class MapSvgColorMapper extends ColorMapper {
     final alpha = (color.a * 255.0).round().clamp(0, 255);
     return replacement.withAlpha(alpha);
   }
+
+  // The SVG loader and its picture cache compare mappers, so a mapper made
+  // again with the same colours, as views do on every build (a replay builds
+  // every frame), must equal the last one. If it didn't, the map would be
+  // loaded and drawn again from scratch.
+  @override
+  bool operator ==(Object other) =>
+      other is MapSvgColorMapper && mapEquals(other.replacements, replacements);
+
+  @override
+  int get hashCode => Object.hashAllUnordered([
+        for (final MapEntry(:key, :value) in replacements.entries)
+          Object.hash(key, value),
+      ]);
 }
diff --git a/test/map_svg_color_mapper_test.dart b/test/map_svg_color_mapper_test.dart
new file mode 100644
index 00000000..cd5684d7
--- /dev/null
+++ b/test/map_svg_color_mapper_test.dart
@@ -0,0 +1,47 @@
+import 'package:flutter/painting.dart';
+import 'package:flutter_svg/flutter_svg.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:icarus/providers/user_preferences_provider.dart';
+import 'package:icarus/widgets/map_svg_color_mapper.dart';
+
+void main() {
+  final palette = MapThemePalette(
+    baseColorValue: 0xFF271406,
+    detailColorValue: 0xFFB27C40,
+    highlightColorValue: 0xFFF08234,
+  );
+
+  // A map widget rebuilt with a mapper made again from the same palette must
+  // keep its loaded picture: flutter_svg reloads whenever the loader changes,
+  // and the loader compares mappers.
+  test('a mapper made again from the same palette gives an equal loader', () {
+    SvgAssetLoader loader(MapSvgColorMapper mapper) =>
+        SvgAssetLoader('assets/maps/ascent_map.svg', colorMapper: mapper);
+
+    for (final make in [
+      MapSvgColorMapper.forPalette,
+      MapSvgColorMapper.revealed,
+    ]) {
+      final again = make(palette.copyWith());
+      expect(again, make(palette));
+      expect(again.hashCode, make(palette).hashCode);
+      expect(loader(again), loader(make(palette)));
+    }
+  });
+
+  test('the order colours were listed in does not matter', () {
+    const red = Color(0xFFFF0000), blue = Color(0xFF0000FF);
+    final one = MapSvgColorMapper(const {1: red, 2: blue});
+    final other = MapSvgColorMapper(const {2: blue, 1: red});
+    expect(other, one);
+    expect(other.hashCode, one.hashCode);
+  });
+
+  test('mappers for different colours differ', () {
+    final other = palette.copyWith(baseColorValue: 0xFF102030);
+    expect(MapSvgColorMapper.forPalette(other),
+        isNot(MapSvgColorMapper.forPalette(palette)));
+    expect(MapSvgColorMapper.revealed(palette),
+        isNot(MapSvgColorMapper.forPalette(palette)));
+  });
+}