[web] Only create one <style> for SelectableRegion (#161682)

We used to create and insert a new `<style>` element for each
`SelectableRegion` widget. That's unnecessary. All we need is one
`<style>` element that contains the style sheets that we want to apply.

Most of this PR is re-working the tests to be able to check that the
issue is actually fixed.

Fixes https://github.com/flutter/flutter/issues/161519

packages/flutter/lib/src/widgets/_platform_selectable_region_context_menu_io.dart

@@ -46,6 +46,12 @@ class PlatformSelectableRegionContextMenu extends StatelessWidget {
   @visibleForTesting
   static RegisterViewFactory? debugOverrideRegisterViewFactory;
 
+  /// Resets the view factory registration to its initial state.
+  @visibleForTesting
+  static void debugResetRegistry() {
+    throw UnimplementedError();
+  }
+
   @override
   Widget build(BuildContext context) => throw UnimplementedError();
 }

packages/flutter/lib/src/widgets/_platform_selectable_region_context_menu_web.dart

@@ -75,6 +75,12 @@ class PlatformSelectableRegionContextMenu extends StatelessWidget {
   @visibleForTesting
   static RegisterViewFactory? debugOverrideRegisterViewFactory;
 
+  /// Resets the view factory registration to its initial state.
+  @visibleForTesting
+  static void debugResetRegistry() {
+    _registeredViewType = null;
+  }
+
   // Registers the view factories for the interceptor widgets.
   static void _register() {
     assert(_registeredViewType == null);
@@ -104,21 +110,21 @@ class PlatformSelectableRegionContextMenu extends StatelessWidget {
   }
 
   static String _registerWebSelectionCallback(_WebSelectionCallBack callback) {
-    _registerViewFactory(_viewType, (int viewId) {
+    // Create css style for _kClassName.
+    final web.HTMLStyleElement styleElement =
+        web.document.createElement('style') as web.HTMLStyleElement;
+    web.document.head!.append(styleElement as JSAny);
+    final web.CSSStyleSheet sheet = styleElement.sheet!;
+    sheet.insertRule(_kClassRule, 0);
+    sheet.insertRule(_kClassSelectionRule, 1);
+
+    _registerViewFactory(_viewType, (int viewId, {Object? params}) {
       final web.HTMLElement htmlElement = web.document.createElement('div') as web.HTMLElement;
       htmlElement
         ..style.width = '100%'
         ..style.height = '100%'
         ..classList.add(_kClassName);
 
-      // Create css style for _kClassName.
-      final web.HTMLStyleElement styleElement =
-          web.document.createElement('style') as web.HTMLStyleElement;
-      web.document.head!.append(styleElement as JSAny);
-      final web.CSSStyleSheet sheet = styleElement.sheet!;
-      sheet.insertRule(_kClassRule, 0);
-      sheet.insertRule(_kClassSelectionRule, 1);
-
       htmlElement.addEventListener(
         'mousedown',
         (web.Event event) {

packages/flutter/test/widgets/html_element_view_test.dart

@@ -8,7 +8,6 @@ library;
 import 'dart:async';
 import 'dart:ui_web' as ui_web;
 
-import 'package:collection/collection.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter/src/widgets/_html_element_view_web.dart'
@@ -17,6 +16,8 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:web/web.dart' as web;
 
+import 'web_platform_view_registry_utils.dart';
+
 final Object _mockHtmlElement = Object();
 Object _mockViewFactory(int id, {Object? params}) {
   return _mockHtmlElement;
@@ -419,101 +420,3 @@ void main() {
     });
   });
 }
-
-typedef FakeViewFactory = ({String viewType, bool isVisible, Function viewFactory});
-
-typedef FakePlatformView = ({int id, String viewType, Object? params, Object htmlElement});
-
-class FakePlatformViewRegistry implements ui_web.PlatformViewRegistry {
-  FakePlatformViewRegistry() {
-    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMethodCallHandler(
-      SystemChannels.platform_views,
-      _onMethodCall,
-    );
-  }
-
-  Set<FakePlatformView> get views => Set<FakePlatformView>.unmodifiable(_views);
-  final Set<FakePlatformView> _views = <FakePlatformView>{};
-
-  final Set<FakeViewFactory> _registeredViewTypes = <FakeViewFactory>{};
-
-  @override
-  bool registerViewFactory(String viewType, Function viewFactory, {bool isVisible = true}) {
-    if (_findRegisteredViewFactory(viewType) != null) {
-      return false;
-    }
-    _registeredViewTypes.add((viewType: viewType, isVisible: isVisible, viewFactory: viewFactory));
-    return true;
-  }
-
-  @override
-  Object getViewById(int viewId) {
-    return _findViewById(viewId)!.htmlElement;
-  }
-
-  FakeViewFactory? _findRegisteredViewFactory(String viewType) {
-    return _registeredViewTypes.singleWhereOrNull(
-      (FakeViewFactory registered) => registered.viewType == viewType,
-    );
-  }
-
-  FakePlatformView? _findViewById(int viewId) {
-    return _views.singleWhereOrNull((FakePlatformView view) => view.id == viewId);
-  }
-
-  Future<dynamic> _onMethodCall(MethodCall call) {
-    return switch (call.method) {
-      'create' => _create(call),
-      'dispose' => _dispose(call),
-      _ => Future<dynamic>.sync(() => null),
-    };
-  }
-
-  Future<dynamic> _create(MethodCall call) async {
-    final Map<dynamic, dynamic> args = call.arguments as Map<dynamic, dynamic>;
-    final int id = args['id'] as int;
-    final String viewType = args['viewType'] as String;
-    final Object? params = args['params'];
-
-    if (_findViewById(id) != null) {
-      throw PlatformException(
-        code: 'error',
-        message: 'Trying to create an already created platform view, view id: $id',
-      );
-    }
-
-    final FakeViewFactory? registered = _findRegisteredViewFactory(viewType);
-    if (registered == null) {
-      throw PlatformException(
-        code: 'error',
-        message: 'Trying to create a platform view of unregistered type: $viewType',
-      );
-    }
-
-    final ui_web.ParameterizedPlatformViewFactory viewFactory =
-        registered.viewFactory as ui_web.ParameterizedPlatformViewFactory;
-
-    _views.add((
-      id: id,
-      viewType: viewType,
-      params: params,
-      htmlElement: viewFactory(id, params: params),
-    ));
-    return null;
-  }
-
-  Future<dynamic> _dispose(MethodCall call) async {
-    final int id = call.arguments as int;
-
-    final FakePlatformView? view = _findViewById(id);
-    if (view == null) {
-      throw PlatformException(
-        code: 'error',
-        message: 'Trying to dispose a platform view with unknown id: $id',
-      );
-    }
-
-    _views.remove(view);
-    return null;
-  }
-}

packages/flutter/test/widgets/selectable_region_context_menu_test.dart

@@ -5,14 +5,15 @@
 @TestOn('browser') // This file contains web-only library.
 library;
 
-import 'dart:js_interop';
-
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:web/web.dart' as web;
 
+import 'web_platform_view_registry_utils.dart';
+
 extension on web.HTMLCollection {
   Iterable<web.Element?> get iterable =>
       Iterable<web.Element?>.generate(length, (int index) => item(index));
@@ -24,47 +25,75 @@ extension on web.CSSRuleList {
 }
 
 void main() {
-  web.HTMLElement? element;
-  PlatformSelectableRegionContextMenu.debugOverrideRegisterViewFactory = (
-    String viewType,
-    Object Function(int viewId) fn, {
-    bool isVisible = true,
-  }) {
-    element = fn(0) as web.HTMLElement;
-    // The element needs to be attached to the document body to receive mouse
-    // events.
-    web.document.body!.append(element! as JSAny);
-  };
-  // This force register the dom element.
-  PlatformSelectableRegionContextMenu(child: const Placeholder());
-  PlatformSelectableRegionContextMenu.debugOverrideRegisterViewFactory = null;
+  late FakePlatformViewRegistry fakePlatformViewRegistry;
+
+  setUp(() {
+    removeAllStyleElements();
+    fakePlatformViewRegistry = FakePlatformViewRegistry();
+    PlatformSelectableRegionContextMenu.debugOverrideRegisterViewFactory =
+        fakePlatformViewRegistry.registerViewFactory;
+  });
+
+  tearDown(() {
+    PlatformSelectableRegionContextMenu.debugOverrideRegisterViewFactory = null;
+    PlatformSelectableRegionContextMenu.debugResetRegistry();
+  });
+
+  testWidgets('DOM element is set up correctly', (WidgetTester tester) async {
+    final int currentViewId = platformViewsRegistry.getNextPlatformViewId();
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: SelectableRegion(
+          selectionControls: EmptyTextSelectionControls(),
+          child: const Placeholder(),
+        ),
+      ),
+    );
+
+    final web.HTMLElement element =
+        fakePlatformViewRegistry.getViewById(currentViewId + 1) as web.HTMLElement;
 
-  test('DOM element is set up correctly', () async {
     expect(element, isNotNull);
-    expect(element!.style.width, '100%');
-    expect(element!.style.height, '100%');
-    expect(element!.classList.length, 1);
-    final String className = element!.className;
+    expect(element.style.width, '100%');
+    expect(element.style.height, '100%');
+    expect(element.classList.length, 1);
 
-    expect(web.document.head!.children.iterable, isNotEmpty);
-    bool foundStyle = false;
-    for (final web.Element? element in web.document.head!.children.iterable) {
-      expect(element, isNotNull);
-      if (element!.tagName != 'STYLE') {
-        continue;
-      }
-      final web.CSSRuleList? rules = (element as web.HTMLStyleElement).sheet?.rules;
-      if (rules != null) {
-        foundStyle = rules.iterable.any((web.CSSRule? rule) => rule!.cssText.contains(className));
-      }
-      if (foundStyle) {
-        break;
-      }
-    }
-    expect(foundStyle, isTrue);
+    final int numberOfStyleElements = getNumberOfStyleElements();
+    expect(numberOfStyleElements, 1);
+  });
+
+  testWidgets('only one <style> is inserted into the DOM', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: ListView(
+          children: <Widget>[
+            SelectableRegion(
+              selectionControls: EmptyTextSelectionControls(),
+              child: const Placeholder(),
+            ),
+            SelectableRegion(
+              selectionControls: EmptyTextSelectionControls(),
+              child: const Placeholder(),
+            ),
+            SelectableRegion(
+              selectionControls: EmptyTextSelectionControls(),
+              child: const Placeholder(),
+            ),
+          ],
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    await tester.pumpAndSettle();
+
+    final int numberOfStyleElements = getNumberOfStyleElements();
+    expect(numberOfStyleElements, 1);
   });
 
   testWidgets('right click can trigger select word', (WidgetTester tester) async {
+    final int currentViewId = platformViewsRegistry.getNextPlatformViewId();
+
     final FocusNode focusNode = FocusNode();
     addTearDown(focusNode.dispose);
     final UniqueKey spy = UniqueKey();
@@ -77,13 +106,16 @@ void main() {
         ),
       ),
     );
+
+    final web.HTMLElement element =
+        fakePlatformViewRegistry.getViewById(currentViewId + 1) as web.HTMLElement;
     expect(element, isNotNull);
 
     focusNode.requestFocus();
     await tester.pump();
 
     // Dispatch right click.
-    element!.dispatchEvent(
+    element.dispatchEvent(
       web.MouseEvent('mousedown', web.MouseEventInit(button: 2, clientX: 200, clientY: 300)),
     );
     final RenderSelectionSpy renderSelectionSpy = tester.renderObject<RenderSelectionSpy>(
@@ -104,6 +136,36 @@ void main() {
   });
 }
 
+void removeAllStyleElements() {
+  final List<web.Element?> styles = web.document.head!.children.iterable.toList();
+  for (final web.Element? element in styles) {
+    if (element!.tagName == 'STYLE') {
+      element.remove();
+    }
+  }
+}
+
+int getNumberOfStyleElements() {
+  expect(web.document.head!.children.iterable, isNotEmpty);
+
+  int count = 0;
+  for (final web.Element? element in web.document.head!.children.iterable) {
+    expect(element, isNotNull);
+    if (element!.tagName != 'STYLE') {
+      continue;
+    }
+    final web.CSSRuleList? rules = (element as web.HTMLStyleElement).sheet?.rules;
+    if (rules != null) {
+      if (rules.iterable.any(
+        (web.CSSRule? rule) => rule!.cssText.contains('web-selectable-region-context-menu'),
+      )) {
+        count++;
+      }
+    }
+  }
+  return count;
+}
+
 class SelectionSpy extends LeafRenderObjectWidget {
   const SelectionSpy({super.key});
 

packages/flutter/test/widgets/web_platform_view_registry_utils.dart

@@ -0,0 +1,107 @@
+// Copyright 2014 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:ui_web' as ui_web;
+
+import 'package:collection/collection.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+typedef FakeViewFactory = ({String viewType, bool isVisible, Function viewFactory});
+
+typedef FakePlatformView = ({int id, String viewType, Object? params, Object htmlElement});
+
+class FakePlatformViewRegistry implements ui_web.PlatformViewRegistry {
+  FakePlatformViewRegistry() {
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger.setMockMethodCallHandler(
+      SystemChannels.platform_views,
+      _onMethodCall,
+    );
+  }
+
+  Set<FakePlatformView> get views => Set<FakePlatformView>.unmodifiable(_views);
+  final Set<FakePlatformView> _views = <FakePlatformView>{};
+
+  final Set<FakeViewFactory> _registeredViewTypes = <FakeViewFactory>{};
+
+  @override
+  bool registerViewFactory(String viewType, Function viewFactory, {bool isVisible = true}) {
+    if (_findRegisteredViewFactory(viewType) != null) {
+      return false;
+    }
+    _registeredViewTypes.add((viewType: viewType, isVisible: isVisible, viewFactory: viewFactory));
+    return true;
+  }
+
+  @override
+  Object getViewById(int viewId) {
+    return _findViewById(viewId)!.htmlElement;
+  }
+
+  FakeViewFactory? _findRegisteredViewFactory(String viewType) {
+    return _registeredViewTypes.singleWhereOrNull(
+      (FakeViewFactory registered) => registered.viewType == viewType,
+    );
+  }
+
+  FakePlatformView? _findViewById(int viewId) {
+    return _views.singleWhereOrNull((FakePlatformView view) => view.id == viewId);
+  }
+
+  Future<dynamic> _onMethodCall(MethodCall call) {
+    return switch (call.method) {
+      'create' => _create(call),
+      'dispose' => _dispose(call),
+      _ => Future<dynamic>.sync(() => null),
+    };
+  }
+
+  Future<dynamic> _create(MethodCall call) async {
+    final Map<dynamic, dynamic> args = call.arguments as Map<dynamic, dynamic>;
+    final int id = args['id'] as int;
+    final String viewType = args['viewType'] as String;
+    final Object? params = args['params'];
+
+    if (_findViewById(id) != null) {
+      throw PlatformException(
+        code: 'error',
+        message: 'Trying to create an already created platform view, view id: $id',
+      );
+    }
+
+    final FakeViewFactory? registered = _findRegisteredViewFactory(viewType);
+    if (registered == null) {
+      throw PlatformException(
+        code: 'error',
+        message: 'Trying to create a platform view of unregistered type: $viewType',
+      );
+    }
+
+    final ui_web.ParameterizedPlatformViewFactory viewFactory =
+        registered.viewFactory as ui_web.ParameterizedPlatformViewFactory;
+
+    _views.add((
+      id: id,
+      viewType: viewType,
+      params: params,
+      htmlElement: viewFactory(id, params: params),
+    ));
+    return null;
+  }
+
+  Future<dynamic> _dispose(MethodCall call) async {
+    final int id = call.arguments as int;
+
+    final FakePlatformView? view = _findViewById(id);
+    if (view == null) {
+      throw PlatformException(
+        code: 'error',
+        message: 'Trying to dispose a platform view with unknown id: $id',
+      );
+    }
+
+    _views.remove(view);
+    return null;
+  }
+}