[web] Fix scroll wheel line delta on Firefox. (#21928)

e2etests/web/regular_integration_tests/lib/scroll_wheel_main.dart

+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+import 'dart:html' as html;
+import 'dart:js_util' as js_util;
+
+import 'package:flutter/material.dart';
+
+void main() => runApp(MyApp());
+
+class MyApp extends StatelessWidget {
+  // This widget is the root of your application.
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Flutter Scroll Wheel Test',
+      theme: ThemeData(
+        // This is the theme of your application.
+        //
+        // Try running your application with "flutter run". You'll see the
+        // application has a blue toolbar. Then, without quitting the app, try
+        // changing the primarySwatch below to Colors.green and then invoke
+        // "hot reload" (press "r" in the console where you ran "flutter run",
+        // or simply save your changes to "hot reload" in a Flutter IDE).
+        // Notice that the counter didn't reset back to zero; the application
+        // is not restarted.
+        primarySwatch: Colors.blue,
+        fontFamily: 'RobotoMono',
+        // This makes the visual density adapt to the platform that you run
+        // the app on. For desktop platforms, the controls will be smaller and
+        // closer together (more dense) than on mobile platforms.
+        visualDensity: VisualDensity.adaptivePlatformDensity,
+      ),
+      home: MyHomePage(title: 'Flutter Scroll Wheel Test'),
+    );
+  }
+}
+
+class MyHomePage extends StatefulWidget {
+  MyHomePage({Key key, this.title}) : super(key: key);
+
+  // This widget is the home page of your application. It is stateful, meaning
+  // that it has a State object (defined below) that contains fields that affect
+  // how it looks.
+
+  // This class is the configuration for the state. It holds the values (in this
+  // case the title) provided by the parent (in this case the App widget) and
+  // used by the build method of the State. Fields in a Widget subclass are
+  // always marked "final".
+
+  final String title;
+
+  @override
+  _MyHomePageState createState() => _MyHomePageState();
+}
+
+class _MyHomePageState extends State<MyHomePage> {
+  int _counter = 0;
+
+  void _incrementCounter() {
+    setState(() {
+      // This call to setState tells the Flutter framework that something has
+      // changed in this State, which causes it to rerun the build method below
+      // so that the display can reflect the updated values. If we changed
+      // _counter without calling setState(), then the build method would not be
+      // called again, and so nothing would appear to happen.
+      _counter++;
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    // This method is rerun every time setState is called, for instance as done
+    // by the _incrementCounter method above.
+    //
+    // The Flutter framework has been optimized to make rerunning build methods
+    // fast, so that you can just rebuild anything that needs updating rather
+    // than having to individually change instances of widgets.
+    return Scaffold(
+      appBar: AppBar(
+        // Here we take the value from the MyHomePage object that was created by
+        // the App.build method, and use it to set our appbar title.
+        title: Text(widget.title),
+      ),
+      body: ListView.builder(
+        itemCount: 1000,
+        itemBuilder: (context, index) => Padding(
+          padding: EdgeInsets.all(20),
+          child: Container(
+            height: 100,
+            color: Colors.lightBlue,
+            child: Center(
+              child: Text("Item $index"),
+            ),
+          ),
+        ),
+      ),
+      floatingActionButton:
+          FloatingActionButton.extended(
+            key: const Key('scroll-button'),
+            onPressed: () {
+              final int centerX = 100; //html.window.innerWidth ~/ 2;
+              final int centerY = 100; //html.window.innerHeight ~/ 2;
+              dispatchMouseWheelEvent(centerX, centerY, DeltaMode.kLine, 0, 1);
+              dispatchMouseWheelEvent(centerX, centerY, DeltaMode.kLine, 0, 1);
+              dispatchMouseWheelEvent(centerX, centerY, DeltaMode.kLine, 0, 1);
+              dispatchMouseWheelEvent(centerX, centerY, DeltaMode.kLine, 0, 1);
+              dispatchMouseWheelEvent(centerX, centerY, DeltaMode.kLine, 0, 1);
+            },
+            label: Text('Scroll'),
+            icon: Icon(Icons.thumb_up),
+          ),
+    );
+  }
+}
+
+
+abstract class DeltaMode {
+  static const int kPixel = 0x00;
+  static const int kLine = 0x01;
+  static const int kPage = 0x02;
+}
+
+html.WheelEvent dispatchMouseWheelEvent(int mouseX, int mouseY,
+    int deltaMode, double deltaX, double deltaY,
+    {bool shiftKeyPressed = false}) {
+  html.EventTarget target = html.document.elementFromPoint(mouseX, mouseY);
+
+  target.dispatchEvent(html.MouseEvent("mouseover",
+    screenX: mouseX,
+    screenY: mouseY,
+    clientX: mouseX,
+    clientY: mouseY,
+  ));
+
+  target.dispatchEvent(html.MouseEvent("mousemove",
+    screenX: mouseX,
+    screenY: mouseY,
+    clientX: mouseX,
+    clientY: mouseY,
+  ));
+
+  html.WheelEvent event = html.WheelEvent('wheel',
+    screenX: mouseX,
+    screenY: mouseY,
+    clientX: mouseX,
+    clientY: mouseY,
+    deltaMode: deltaMode,
+    deltaX : deltaX,
+    deltaY : deltaY,
+    shiftKey: shiftKeyPressed,
+  );
+  target.dispatchEvent(event);
+  return event;
+}

e2etests/web/regular_integration_tests/test_driver/scroll_wheel_integration.dart

+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:html' as html;
+import 'dart:js_util' as js_util;
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:regular_integration_tests/scroll_wheel_main.dart' as app;
+
+import 'package:integration_test/integration_test.dart';
+
+void main() {
+  final IntegrationTestWidgetsFlutterBinding binding = IntegrationTestWidgetsFlutterBinding.ensureInitialized() as IntegrationTestWidgetsFlutterBinding;
+
+  testWidgets('Test mousewheel scroll by line',
+      (WidgetTester tester) async {
+    app.main();
+    await tester.pumpAndSettle();
+
+    final Finder finder = find.byKey(const Key('scroll-button'));
+    expect(finder, findsOneWidget);
+    await tester.tap(find.byKey(const Key('scroll-button')));
+    await tester.pumpAndSettle();
+    await tester.tap(find.byKey(const Key('scroll-button')));
+    await tester.pumpAndSettle();
+
+    // TODO: enable screenshot when
+    //  https://github.com/flutter/flutter/issues/68502 is resolved.
+    await binding.takeScreenshot('wheel_scroll_by_line');
+  });
+}

e2etests/web/regular_integration_tests/test_driver/scroll_wheel_integration_test.dart

+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:regular_integration_tests/screenshot_support.dart' as test;
+
+Future<void> main() async {
+  // TODO: switch to screenshot when
+  //  https://github.com/flutter/flutter/issues/68502 is resolved.
+  // await test.runTestWithScreenshots();
+}

lib/web_ui/lib/src/engine/pointer_binding.dart

@@ -227,6 +227,8 @@ abstract class _BaseAdapter {
 }
 
 mixin _WheelEventListenerMixin on _BaseAdapter {
+  static double? _defaultScrollLineHeight;
+
   List<ui.PointerData> _convertWheelEventToPointerData(
     html.WheelEvent event
   ) {
@@ -240,8 +242,9 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
     double deltaY = event.deltaY as double;
     switch (event.deltaMode) {
       case domDeltaLine:
-        deltaX *= 32.0;
-        deltaY *= 32.0;
+        _defaultScrollLineHeight ??= _computeDefaultScrollLineHeight();
+        deltaX *= _defaultScrollLineHeight!;
+        deltaY *= _defaultScrollLineHeight!;
         break;
       case domDeltaPage:
         deltaX *= ui.window.physicalSize.width;
@@ -251,6 +254,7 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
       default:
         break;
     }
+
     final List<ui.PointerData> data = <ui.PointerData>[];
     _pointerDataConverter.convert(
       data,
@@ -285,6 +289,27 @@ mixin _WheelEventListenerMixin on _BaseAdapter {
       ]
     );
   }
+
+  /// For browsers that report delta line instead of pixels such as FireFox
+  /// compute line height using the default font size.
+  ///
+  /// Use Firefox to test this code path.
+  double _computeDefaultScrollLineHeight() {
+    const double kFallbackFontHeight = 16.0;
+    final html.DivElement probe = html.DivElement();
+    probe.style
+        ..fontSize = 'initial'
+        ..display = 'none';
+    html.document.body!.append(probe);
+    String fontSize = probe.getComputedStyle().fontSize;
+    double? res;
+    if (fontSize.contains('px')) {
+       fontSize = fontSize.replaceAll('px', '');
+       res = double.tryParse(fontSize);
+    }
+    probe.remove();
+    return res == null ? kFallbackFontHeight : res / 4.0;
+  }
 }
 
 @immutable