Handle hasStrings on web (#132093)

By default, Flutter web uses the browser's built-in context menu.

<img width="200" src="https://github.com/flutter/flutter/assets/389558/990f99cb-bc38-40f1-9e88-8839bc342da5" />

As of [recently](https://github.com/flutter/engine/pull/38682), it's possible to use a Flutter-rendered context menu like the other platforms.

```dart
void main() {
  runApp(const MyApp());
  BrowserContextMenu.disableContextMenu();
}
```

But there is a bug (https://github.com/flutter/flutter/issues/129692) that the Paste button is missing and never shows up.

<img width="284" alt="Screenshot 2023-08-07 at 2 39 03 PM" src="https://github.com/flutter/flutter/assets/389558/f632be25-28b1-4e2e-98f7-3bb443f077df">

The reason why it's missing is that Flutter first checks if there is any pasteable text on the clipboard before deciding to show the Paste button using the `hasStrings` platform channel method, but that was never implemented for web ([original hasStrings PR](https://github.com/flutter/flutter/pull/87678)).

So let's just implement hasStrings for web?  No, because Chrome shows a permissions prompt when the clipboard is accessed, and there is [no browser clipboard API](https://developer.mozilla.org/en-US/docs/Web/API/Clipboard_API) to avoid it.  The prompt will show immediately when the EditableText is built, not just when the Paste button is pressed.

<img width="200" src="https://github.com/flutter/flutter/assets/389558/5abdb160-1b13-4f1a-87e1-4653ca19d73e" />

### This PR's solution

Instead, before implementing hasStrings for web, this PR disables the hasStrings check for web.  The result is that users will always see a paste button, even in the (unlikely) case that they have nothing pasteable on the clipboard.  However, they will not see a permissions dialog until they actually click the Paste button.  Subsequent pastes don't show the permission dialog.

<details>

<summary>Video of final behavior with this PR</summary>

https://github.com/flutter/flutter/assets/389558/ed16c925-8111-44a7-99e8-35a09d682748

</details>

I think this will be the desired behavior for the vast majority of app developers.  Those that want different behavior can use hasStrings themselves, which will be implemented in https://github.com/flutter/engine/pull/43360.

### References

Fixes https://github.com/flutter/flutter/issues/129692
Engine PR to be merged after this: https://github.com/flutter/engine/pull/43360

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

@@ -2102,7 +2102,13 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
   final GlobalKey _editableKey = GlobalKey();
 
   /// Detects whether the clipboard can paste.
-  final ClipboardStatusNotifier clipboardStatus = ClipboardStatusNotifier();
+  final ClipboardStatusNotifier clipboardStatus = kIsWeb
+      // Web browsers will show a permission dialog when Clipboard.hasStrings is
+      // called. In an EditableText, this will happen before the paste button is
+      // clicked, often before the context menu is even shown. To avoid this
+      // poor user experience, always show the paste button on web.
+      ? _WebClipboardStatusNotifier()
+      : ClipboardStatusNotifier();
 
   /// Detects whether the Live Text input is enabled.
   ///
@@ -5561,3 +5567,18 @@ class _GlyphHeights {
   /// The glyph height of the last line.
   final double end;
 }
+
+/// A [ClipboardStatusNotifier] whose [value] is hardcoded to
+/// [ClipboardStatus.pasteable].
+///
+/// Useful to avoid showing a permission dialog on web, which happens when
+/// [Clipboard.hasStrings] is called.
+class _WebClipboardStatusNotifier extends ClipboardStatusNotifier {
+  @override
+  ClipboardStatus value = ClipboardStatus.pasteable;
+
+  @override
+  Future<void> update() {
+    return Future<void>.value();
+  }
+}

packages/flutter/test/material/text_field_test.dart

@@ -14154,7 +14154,9 @@ void main() {
     expect(calledGetData, false);
     // hasStrings is checked in order to decide if the content can be pasted.
     expect(calledHasStrings, true);
-  });
+  },
+    skip: kIsWeb, // [intended] web doesn't call hasStrings.
+  );
 
   testWidgets('TextField changes mouse cursor when hovered', (WidgetTester tester) async {
     await tester.pumpWidget(

packages/flutter/test/widgets/editable_text_test.dart

@@ -63,11 +63,10 @@ TextEditingValue collapsedAtEnd(String text) {
 }
 
 void main() {
+  setUp(() async {
     final MockClipboard mockClipboard = MockClipboard();
     TestWidgetsFlutterBinding.ensureInitialized()
       .defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, mockClipboard.handleMethodCall);
-
-  setUp(() async {
     debugResetSemanticsIdCounter();
     controller = TextEditingController();
     // Fill the clipboard so that the Paste option is available in the text
@@ -76,6 +75,8 @@ void main() {
   });
 
   tearDown(() {
+    TestWidgetsFlutterBinding.ensureInitialized()
+      .defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, null);
     controller.dispose();
   });
 
@@ -2156,9 +2157,11 @@ void main() {
 
     final TextSelection copySelectionRange = localController.selection;
 
+    expect(find.byType(TextSelectionToolbar), findsNothing);
     state.showToolbar();
     await tester.pumpAndSettle();
 
+    expect(find.byType(TextSelectionToolbar), findsOneWidget);
     expect(find.text('Copy'), findsOneWidget);
 
     await tester.tap(find.text('Copy'));
@@ -16603,6 +16606,56 @@ testWidgets('Floating cursor ending with selection', (WidgetTester tester) async
   },
     skip: kIsWeb, // [intended]
   );
+
+  group('hasStrings', () {
+    late int calls;
+    setUp(() {
+      calls = 0;
+      TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(SystemChannels.platform, (MethodCall methodCall) {
+          if (methodCall.method == 'Clipboard.hasStrings') {
+            calls += 1;
+          }
+          return Future<void>.value();
+        });
+    });
+    tearDown(() {
+      TestWidgetsFlutterBinding.ensureInitialized()
+        .defaultBinaryMessenger.setMockMethodCallHandler(SystemChannels.platform, null);
+    });
+
+    testWidgets('web avoids the paste permissions prompt by not calling hasStrings', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        MaterialApp(
+          home: EditableText(
+            backgroundCursorColor: Colors.grey,
+            controller: TextEditingController(),
+            focusNode: focusNode,
+            obscureText: true,
+            toolbarOptions: const ToolbarOptions(
+              copy: true,
+              cut: true,
+              paste: true,
+              selectAll: true,
+            ),
+            style: textStyle,
+            cursorColor: cursorColor,
+            selectionControls: materialTextSelectionControls,
+          ),
+        ),
+      );
+
+      expect(calls, equals(kIsWeb ? 0 : 1));
+
+      // Long-press to bring up the context menu.
+      final Finder textFinder = find.byType(EditableText);
+      await tester.longPress(textFinder);
+      tester.state<EditableTextState>(textFinder).showToolbar();
+      await tester.pumpAndSettle();
+
+      expect(calls, equals(kIsWeb ? 0 : 2));
+    });
+  });
 }
 
 class UnsettableController extends TextEditingController {