moving the overlaying the text cursor

text_cursor_overlay.dart

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: const Scaffold(
        body: Center(
          child: Demo(),
        ),
      ),
    );
  }
}

class Demo extends StatefulWidget {
  const Demo({super.key});

  @override
  State<Demo> createState() => _DemoState();
}

class _DemoState extends State<Demo> {
  late final _node = FocusNode()
    ..addListener(() {
      setState(() {});
    });

  late final _controller = TextEditingController()
    ..addListener(() {
      _updateCursor();
    });

  var _rect = Rect.zero;

  @override
  void dispose() {
    _node.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      fit: StackFit.passthrough,
      children: [
        TextField(
          focusNode: _node,
          controller: _controller,
          decoration: const InputDecoration(
            labelText: 'Name',
            hintText: 'Enter your name',
            border: OutlineInputBorder(),
          ),
          // onChanged: _changed,
        ),
        if (_node.hasFocus)
          Positioned(
            top: 15 + _rect.top + 10,
            left: 12 + _rect.left,
            child: Container(
              width: _rect.width,
              height: _rect.height,
              color: Colors.red,
            ),
          ),
      ],
    );
  }

  void _updateCursor() {
    final editable = context.findRenderObject()?.findChildOfType<RenderEditable>();
    if (editable != null) {
      WidgetsBinding.instance.addPostFrameCallback((_) {
        setState(() {
          final selection = _controller.selection;
          _rect = editable.getLocalRectForCaret(selection.extent);
        });
      });
    }
  }
}

extension on RenderObject {
  T? findChildOfType<T extends RenderBox>() {
    if (this is T) {
      return this as T;
    }
    T? result;
    visitChildren((child) {
      result ??= child.findChildOfType<T>();
    });
    return result;
  }
}