flutter/flutter#34416

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MultilineTextApp()
    );
  }
}

class MultilineTextApp extends StatelessWidget {
  static const TITLE = 'Multiline Test';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: TITLE,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
          appBar: AppBar(
            title: Text(TITLE),
          ),
          body: TextFormField(
            decoration: InputDecoration(
              filled: true,
            ),
            keyboardType: TextInputType.multiline,
            style: TextStyle(color: Colors.red.withOpacity(0.6)),
            maxLines: null,
            // strutStyle: StrutStyle(),  // uncomment to fix
          )),
      builder: (BuildContext context, Widget child) {
        return MediaQuery(
          data: MediaQuery.of(context).copyWith(
            textScaleFactor: 2,
          ),
          child: child,
        );
      },
    );
  }
}