import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  createState() => MyWidgetState();
}

class MyWidgetState extends State<MyWidget> {
  TextEditingController _cont1;
  TextEditingController _cont2;

  @override
  initState() {
    _cont1 = TextEditingController();
    _cont2 = TextEditingController();

    _cont1.text = 'Text selection works here';
    _cont2.text = 'Text selection does not work\nwhen text is multi-line.';
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        TextField(controller: _cont1),
        TextField(controller: _cont2, maxLines: null),
      ],
    );
  }
}