Fix getOffsetForCaret to return correct value if contains widget span (#98542)

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

@@ -41,6 +41,9 @@ abstract class PlaceholderSpan extends InlineSpan {
     TextStyle? style,
   }) : super(style: style);
 
+  /// The unicode character to represent a placeholder.
+  static const String placeholderCodeUnit = '\uFFFC';
+
   /// How the placeholder aligns vertically with the text.
   ///
   /// See [ui.PlaceholderAlignment] for details on each mode.
@@ -57,7 +60,7 @@ abstract class PlaceholderSpan extends InlineSpan {
   @override
   void computeToPlainText(StringBuffer buffer, {bool includeSemanticsLabels = true, bool includePlaceholders = true}) {
     if (includePlaceholders) {
-      buffer.write('\uFFFC');
+      buffer.write(placeholderCodeUnit);
     }
   }
 

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

@@ -739,7 +739,7 @@ class TextPainter {
   // Get the Rect of the cursor (in logical pixels) based off the near edge
   // of the character upstream from the given string offset.
   Rect? _getRectFromUpstream(int offset, Rect caretPrototype) {
-    final String flattenedText = _text!.toPlainText(includePlaceholders: false);
+    final String flattenedText = _text!.toPlainText(includeSemanticsLabels: false);
     final int? prevCodeUnit = _text!.codeUnitAt(max(0, offset - 1));
     if (prevCodeUnit == null)
       return null;
@@ -789,7 +789,7 @@ class TextPainter {
   // Get the Rect of the cursor (in logical pixels) based off the near edge
   // of the character downstream from the given string offset.
   Rect? _getRectFromDownstream(int offset, Rect caretPrototype) {
-    final String flattenedText = _text!.toPlainText(includePlaceholders: false);
+    final String flattenedText = _text!.toPlainText(includeSemanticsLabels: false);
     // We cap the offset at the final index of the _text.
     final int? nextCodeUnit = _text!.codeUnitAt(min(offset, flattenedText.length - 1));
     if (nextCodeUnit == null)

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

@@ -139,7 +139,8 @@ class WidgetSpan extends PlaceholderSpan {
 
   @override
   int? codeUnitAtVisitor(int index, Accumulator offset) {
-    return null;
+    offset.increment(1);
+    return PlaceholderSpan.placeholderCodeUnit.codeUnitAt(0);
   }
 
   @override

packages/flutter/test/painting/text_painter_test.dart

@@ -37,6 +37,24 @@ void main() {
     expect(caretOffset.dx, painter.width);
   });
 
+  test('TextPainter caret test with WidgetSpan', () {
+    // Regression test for https://github.com/flutter/flutter/issues/98458.
+    final TextPainter painter = TextPainter()
+      ..textDirection = TextDirection.ltr;
+
+    painter.text = const TextSpan(children: <InlineSpan>[
+      TextSpan(text: 'before'),
+      WidgetSpan(child: Text('widget')),
+      TextSpan(text: 'after'),
+    ]);
+    painter.setPlaceholderDimensions(const <PlaceholderDimensions>[
+      PlaceholderDimensions(size: Size(50, 30), baselineOffset: 25, alignment: ui.PlaceholderAlignment.bottom),
+    ]);
+    painter.layout();
+    final Offset caretOffset = painter.getOffsetForCaret(ui.TextPosition(offset: painter.text!.toPlainText().length), ui.Rect.zero);
+    expect(caretOffset.dx, painter.width);
+  }, skip: isBrowser && !isCanvasKit); // https://github.com/flutter/flutter/issues/56308
+
   test('TextPainter null text test', () {
     final TextPainter painter = TextPainter()
       ..textDirection = TextDirection.ltr;