import 'package:flutter/material.dart';

void main() => runApp(const App());

class App extends StatelessWidget {
  const App();

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: SizedBox.expand(
            child: TextField(maxLines: null),
          ),
        ),
      ),
    );
  }
}

import 'package:flutter/material.dart';

void main() => runApp(const App());

class App extends StatefulWidget {
  const App();

  @override
  State<App> createState() => _AppState();
}

class _AppState extends State<App> {
  final _controller = MyTextEditingController();

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: SizedBox.expand(
            child: TextField(
              controller: _controller,
              maxLines: null,
              autofocus: true,
            ),
          ),
        ),
      ),
    );
  }
}

class MyTextEditingController extends TextEditingController {
  MyTextEditingController() {
    _prepareTextSpans();
  }

  final List<TextSpan> _textSpans1 = [];
  final List<TextSpan> _textSpans2 = [];
  String _text = '';
  TextSelection? _prevSelection;
  bool _flag = false;

  @override
  TextSpan buildTextSpan({
    required BuildContext context,
    TextStyle? style,
    required bool withComposing,
  }) {
    if (selection != _prevSelection) {
      _flag = !_flag;
    }
    _prevSelection = selection;

    WidgetsBinding.instance.addPostFrameCallback((_) {
      value = value.copyWith(text: _text);
    });

    return TextSpan(
      style: style,
      children: _flag ? _textSpans1 : _textSpans2,
    );
  }

  void _prepareTextSpans() {
    for (var i = 0; i < 10000; i++) {
      for (var j = 0; j < 4; j++) {
        _textSpans1.add(
          TextSpan(
            text: j.isEven ? 'aaaaa' : 'bbbbb',
            style: j.isEven ? const TextStyle(color: Colors.orange) : null,
          ),
        );
        _textSpans2.add(
          TextSpan(
            text: j.isEven ? 'aaaaa' : 'bbbbb',
            style: j.isEven ? null : const TextStyle(color: Colors.orange),
          ),
        );
      }
      _textSpans1.add(const TextSpan(text: '\n'));
      _textSpans2.add(const TextSpan(text: '\n'));
    }
    _text = _textSpans1.map((v) => v.text).join();
  }
}