fix(cursor): align Web hotspots with rounded cursor images

flutter/lib/models/model.dart

@@ -2920,8 +2920,8 @@ class CursorData {
           img2.encodePng(
             img2.copyResize(
               image,
-              width: (width * scale).toInt(),
+              width: isWeb ? (width * scale).round() : (width * scale).toInt(),
-              height: (height * scale).toInt(),
+              height: isWeb ? (height * scale).round() : (height * scale).toInt(),
               interpolation: img2.Interpolation.average,
             ),
           ),
@@ -2932,6 +2932,11 @@ class CursorData {
     this.scale = scale;
     hotx = hotxOrigin * scale;
     hoty = hotyOrigin * scale;
+    if (isWeb) {
+      // CSS hotspots must follow the actual rounded PNG dimensions.
+      hotx = hotxOrigin * (width * scale).round() / width;
+      hoty = hotyOrigin * (height * scale).round() / height;
+    }
     return scale;
   }
 

flutter/lib/web/custom_cursor.dart

@@ -52,8 +52,8 @@ class CursorManager {
         'cursor',
         jsonEncode({
           'url': cursorData.url,
-          'hotx': cursorData.hotX.toInt(),
+          'hotx': cursorData.hotX.round(),
-          'hoty': cursorData.hotY.toInt(),
+          'hoty': cursorData.hotY.round(),
         })
       ]);
     }
@@ -116,8 +116,8 @@ MouseCursor buildCursorOfCache(
       CursorManager.instance.registerCursor(CursorData(
           key: key,
           url: 'data:image/rgba;base64,${base64Encode(data)}',
-          width: (cache.width * cache.scale).toInt(),
+          width: (cache.width * cache.scale).round(),
-          height: (cache.height * cache.scale).toInt(),
+          height: (cache.height * cache.scale).round(),
           hotX: cache.hotx,
           hotY: cache.hoty));
       cursor.addKey(key);

flutter/test/cursor_web_test.dart

@@ -0,0 +1,74 @@
+@TestOn('browser')
+library;
+
+import 'dart:convert';
+import 'dart:js' as js;
+import 'dart:typed_data';
+import 'dart:ui' as ui;
+
+import 'package:flutter_hbb/models/model.dart' as model;
+import 'package:flutter_hbb/web/custom_cursor.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:image/image.dart' as img;
+
+class _CursorModel extends Fake implements model.CursorModel {
+  @override
+  final Set<String> cachedKeys = {};
+  @override
+  void addKey(String key) => cachedKeys.add(key);
+}
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  test('Web cursor aligns CSS hotspots with rounded PNG dimensions', () async {
+    Map<String, dynamic>? registered;
+    final original = js.context['setByName'];
+    js.context['setByName'] = js.allowInterop((String name, String value) {
+      expect(name, 'cursor');
+      registered = jsonDecode(value) as Map<String, dynamic>;
+    });
+    addTearDown(() => js.context['setByName'] = original);
+    final recorder = ui.PictureRecorder();
+    ui.Canvas(recorder).drawColor(const ui.Color(0xffffffff), ui.BlendMode.src);
+    final picture = recorder.endRecording();
+    final nativeImage = await picture.toImage(48, 48);
+    picture.dispose();
+    addTearDown(nativeImage.dispose);
+    for (final (hotspot, scale, side, expected) in [
+      ((7.0, 7.0), 633 / 1600, 19, (3, 3)),
+      ((21.0, 23.0), 633 / 1600, 19, (8, 9)),
+      ((22.0, 22.0), 633 / 1600, 19, (9, 9)),
+      ((21.0, 23.0), 0.05, 12, (5, 6)),
+      ((21.0, 23.0), 0.5, 24, (11, 12)),
+      ((7.0, 7.0), 1.0, 48, (7, 7)),
+    ]) {
+      final cache = _data(nativeImage, hotspot);
+      final session =
+          buildCursorOfCache(_CursorModel(), scale, cache).createSession(1);
+      await session.activate();
+      final uri = Uri.parse(registered!['url'] as String);
+      final bitmap = img.decodePng(uri.data!.contentAsBytes())!;
+      expect((bitmap.width, bitmap.height), (side, side));
+      expect((registered!['hotx'], registered!['hoty']), expected);
+      session.dispose();
+      await deleteCustomCursor(cache.updateGetKey(scale));
+    }
+  });
+}
+
+model.CursorData _data(ui.Image nativeImage, (double, double) hotspot) {
+  final image = img.Image(width: 48, height: 48, numChannels: 4);
+  img.fill(image, color: img.ColorRgba8(255, 255, 255, 255));
+  return model.CursorData(
+    peerId: 'web-cursor-test',
+    id: '$hotspot',
+    image: image,
+    nativeImage: nativeImage,
+    scale: 1,
+    data: Uint8List.fromList(img.encodePng(image)),
+    hotxOrigin: hotspot.$1,
+    hotyOrigin: hotspot.$2,
+    width: 48,
+    height: 48,
+  );
+}