Random letter shows before web finished loading (Flutter Web)

I have very interesting issue. I tried looking for any help but couldn't find anyone facing it.

I had an issue on my regular build therefore I cut everything off and left only plain version of it. My main.dart:

import 'package:flutter_web/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(home: Scaffold(body: Text('hi'),));
  }
}

Issue is visible even here! It shows letter r in black, then color changes in red and then MaterialApp is returned. Video of it - https://gph.is/g/4LNMY8p

What's happening there? How can I remove this letter from being shown?