Substitute a replacement character for invalid UTF-16 text in a TextSpan (#84887)

packages/flutter/lib/src/painting/text_span.dart

@@ -244,8 +244,20 @@ class TextSpan extends InlineSpan implements HitTestTarget, MouseTrackerAnnotati
     final bool hasStyle = style != null;
     if (hasStyle)
       builder.pushStyle(style!.getTextStyle(textScaleFactor: textScaleFactor));
-    if (text != null)
-      builder.addText(text!);
+    if (text != null) {
+      try {
+        builder.addText(text!);
+      } on ArgumentError catch (exception, stack) {
+        FlutterError.reportError(FlutterErrorDetails(
+          exception: exception,
+          stack: stack,
+          library: 'painting library',
+          context: ErrorDescription('while building a TextSpan'),
+        ));
+        // Use a Unicode replacement character as a substitute for invalid text.
+        builder.addText('\uFFFD');
+      }
+    }
     if (children != null) {
       for (final InlineSpan child in children!) {
         assert(child != null);

packages/flutter/test/painting/text_painter_test.dart

@@ -4,6 +4,7 @@
 
 import 'dart:ui' as ui;
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
@@ -952,6 +953,24 @@ void main() {
       expect(glyphBox, newGlyphBox);
     });
   }, skip: isBrowser);
+
+  test('TextPainter handles invalid UTF-16', () {
+    Object? exception;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      exception = details.exception;
+    };
+
+    final TextPainter painter = TextPainter()
+      ..textDirection = TextDirection.ltr;
+
+    const String text = 'Hello\uD83DWorld';
+    const double fontSize = 20.0;
+    painter.text = const TextSpan(text: text, style: TextStyle(fontSize: fontSize));
+    painter.layout();
+    // The layout should include one replacement character.
+    expect(painter.width, equals(fontSize));
+    expect(exception, isNotNull);
+  }, skip: kIsWeb);
 }
 
 class MockCanvas extends Fake implements Canvas {