Keep resized Web cursors within a proportional minimum

flutter/lib/models/model.dart

@@ -2906,6 +2906,9 @@ class CursorData {
       }
     }
 
+    // Web's long-edge minimum can round a thin axis below one raster pixel.
+    final webWidth = max(1, (width * scale).round());
+    final webHeight = max(1, (height * scale).round());
     if (resizeImage && _doubleToInt(oldScale) != _doubleToInt(scale)) {
       if (isWindows) {
         data = img2
@@ -2921,8 +2924,8 @@ class CursorData {
           img2.encodePng(
             img2.copyResize(
               image,
-              width: isWeb ? (width * scale).round() : (width * scale).toInt(),
-              height: isWeb ? (height * scale).round() : (height * scale).toInt(),
+              width: isWeb ? webWidth : (width * scale).toInt(),
+              height: isWeb ? webHeight : (height * scale).toInt(),
               interpolation: img2.Interpolation.average,
             ),
           ),
@@ -2935,8 +2938,8 @@ class CursorData {
     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;
+      hotx = hotxOrigin * webWidth / width;
+      hoty = hotyOrigin * webHeight / height;
     }
     return scale;
   }

flutter/lib/web/custom_cursor.dart

@@ -1,5 +1,6 @@
 import 'dart:convert';
 import 'dart:js' as js;
+import 'dart:math';
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
@@ -52,8 +53,9 @@ class CursorManager {
         'cursor',
         jsonEncode({
           'url': cursorData.url,
-          'hotx': cursorData.hotX.round(),
-          'hoty': cursorData.hotY.round(),
+          // Rounding must keep the hotspot inside even a one-pixel raster.
+          'hotx': cursorData.hotX.round().clamp(0, cursorData.width - 1),
+          'hoty': cursorData.hotY.round().clamp(0, cursorData.height - 1),
         })
       ]);
     }
@@ -104,7 +106,9 @@ MouseCursor buildCursorOfCache(
   if (cache == null) {
     return MouseCursor.defer;
   } else {
-    final key = cache.updateGetKey(scale);
+    // A short-edge minimum can enlarge thin artwork beyond CSS cursor limits.
+    // Keep unzoomed images unchanged and use the long edge when resizing.
+    final key = cache.updateGetKey(scale, useLegacyMinimum: scale == 1.0);
     if (!cursor.cachedKeys.contains(key)) {
       // data should be checked here, because it may be changed after `updateGetKey()`
       final data = cache.data;
@@ -116,8 +120,8 @@ MouseCursor buildCursorOfCache(
       CursorManager.instance.registerCursor(CursorData(
           key: key,
           url: 'data:image/rgba;base64,${base64Encode(data)}',
-          width: (cache.width * cache.scale).round(),
-          height: (cache.height * cache.scale).round(),
+          width: max(1, (cache.width * cache.scale).round()),
+          height: max(1, (cache.height * cache.scale).round()),
           hotX: cache.hotx,
           hotY: cache.hoty));
       cursor.addKey(key);

flutter/test/cursor_web_test.dart

@@ -22,6 +22,7 @@ import 'package:provider/provider.dart';
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
   _alphaTests();
+  _thinCursorTests();
   for (final style in [kRemoteViewStyleAdaptive, kRemoteViewStyleCustom]) {
     for (final dpr in [1.0, 2.0]) {
       testWidgets('ImagePaint Web $style zoom off DPR $dpr keeps source size',
@@ -55,6 +56,42 @@ void main() {
   });
 }
 
+void _thinCursorTests() {
+  for (final style in [kRemoteViewStyleAdaptive, kRemoteViewStyleCustom]) {
+    for (final zoom in [false, true]) {
+      testWidgets('ImagePaint Web $style thin cursor zoom=$zoom', (tester) =>
+          tester.runAsync(() => _checkPolicy(tester, style, 1,
+              zoom: zoom, source: (64, 4), hotspot: (32, 2),
+              expectedSize: zoom ? (32, 2) : (64, 4),
+              expectedHotspot: zoom ? (16, 1) : (32, 2))));
+    }
+  }
+  test('Web thin cursors keep a raster pixel and an in-bounds hotspot', () async {
+    final registered = _captureCursor();
+    final canvas = _Canvas(kRemoteViewStyleAdaptive);
+    addTearDown(canvas.dispose);
+    final ffi = _FFI(canvas);
+    for (final (source, hotspot, scale, size, expected) in [
+      ((4, 64), (2.0, 32.0), 0.5, (2, 32), (1, 16)),
+      ((2, 128), (1.0, 64.0), 0.01, (1, 12), (0, 6)),
+      ((128, 2), (64.0, 1.0), 0.01, (12, 1), (6, 0)),
+    ]) {
+      final cursor = await _loadCursor(ffi, '$source',
+          source: source, hotspot: hotspot);
+      for (final factor in [1.0, scale, 1.0]) {
+        final session =
+            buildCursorOfCache(cursor, factor, cursor.cache).createSession(1);
+        await session.activate();
+        session.dispose();
+        final png = img.decodePng(Uri.parse(registered['url']).data!.contentAsBytes())!;
+        expect((png.width, png.height), factor == 1 ? source : size);
+        expect((registered['hotx'], registered['hoty']),
+            factor == 1 ? (hotspot.$1, hotspot.$2) : expected);
+      }
+    }
+  });
+}
+
 class _Image extends ChangeNotifier implements model.ImageModel {
   @override
   bool get useTextureRender => false;
@@ -114,17 +151,18 @@ class _FFI extends Fake implements model.FFI {
 
 Future<model.CursorModel> _loadCursor(model.FFI ffi, String id,
     {List<int> pixel = const [255, 255, 255, 255],
+    (int, int) source = (48, 48),
     (double, double) hotspot = (7, 9),
     double? pixelRatio = 1}) async {
   final cursor = model.CursorModel(WeakReference(ffi))..id = id;
   await cursor.updateCursorData({
     'id': id,
-    'width': '48',
-    'height': '48',
+    'width': '${source.$1}',
+    'height': '${source.$2}',
     'hotx': '${hotspot.$1}',
     'hoty': '${hotspot.$2}',
     if (pixelRatio != null) 'scale': '$pixelRatio',
-    'colors': jsonEncode([for (var i = 0; i < 48 * 48; i++) ...pixel]),
+    'colors': jsonEncode([for (var i = 0; i < source.$1 * source.$2; i++) ...pixel]),
   });
   addTearDown(() async {
     // Keep the session owner alive across asynchronous image decoding.
@@ -161,12 +199,16 @@ Map<String, dynamic> _captureCursor() {
   return registered;
 }
 
-Future<void> _checkPolicy(WidgetTester tester, String style, double dpr) async {
+Future<void> _checkPolicy(WidgetTester tester, String style, double dpr,
+    {bool zoom = false, (int, int) source = (48, 48),
+    (double, double) hotspot = (7, 9), (int, int) expectedSize = (48, 48),
+    (int, int) expectedHotspot = (7, 9)}) async {
   final registered = _captureCursor();
   final canvas = _Canvas(style);
   addTearDown(canvas.dispose);
   final ffi = _FFI(canvas);
-  final cursor = await _loadCursor(ffi, '$style-$dpr');
+  final cursor = await _loadCursor(ffi, '$style-$dpr-$zoom-$source',
+      source: source, hotspot: hotspot);
   await tester.pumpWidget(MediaQuery(
     data: MediaQueryData(devicePixelRatio: dpr),
     child: MultiProvider(
@@ -178,7 +220,7 @@ Future<void> _checkPolicy(WidgetTester tester, String style, double dpr) async {
         child: ImagePaint(
             ffi: ffi,
             id: 'web-cursor-test',
-            zoomCursor: false.obs,
+            zoomCursor: zoom.obs,
             cursorOverImage: true.obs,
             keyboardEnabled: true.obs,
             remoteCursorMoved: false.obs)),
@@ -192,8 +234,8 @@ Future<void> _checkPolicy(WidgetTester tester, String style, double dpr) async {
   await tester.pumpWidget(const SizedBox.shrink());
   final png =
       img.decodePng(Uri.parse(registered['url']).data!.contentAsBytes())!;
-  expect((png.width, png.height), (48, 48));
-  expect((registered['hotx'], registered['hoty']), (7, 9));
+  expect((png.width, png.height), expectedSize);
+  expect((registered['hotx'], registered['hoty']), expectedHotspot);
 }
 
 void _alphaTests() {