Demonstration of "my" keyboard bug

main.dart

import 'package:flutter/material.dart';

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

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

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  final _focus = FocusNode();
  final _controller = TextEditingController();
  bool _allowLetters = true;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Keyboard bug',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.blue),
        useMaterial3: true,
      ),
      home: Scaffold(
        appBar: AppBar(
          title: const Text('Keyboard bug'),
          backgroundColor: Colors.blue,
        ),
        body: Padding(
          padding: const EdgeInsets.all(16.0),
          child: Column(
            children: [
              Text(
                  style: Theme.of(context).textTheme.bodySmall,
                  'Please tap into the input field below, hide the keyboard '
                  'and press any key on your keyboard. When writing this app '
                  'the on Screen keyboard opens which is not an expected '
                  'behavior.\n\nAttentional this does not happen when the '
                  'keyboardType is number.\n\nThis is a relevant case for '
                  'devices with physical scanners.'),
              TextField(
                focusNode: _focus,
                controller: _controller,
                keyboardType:
                    _allowLetters ? TextInputType.text : TextInputType.number,
                autofocus: true,
                decoration: InputDecoration(
                  hintText: 'Room for your input',
                  suffixIcon: IconButton(
                    icon: Icon(
                      _allowLetters
                          ? Icons.font_download_outlined
                          : Icons.font_download_off_outlined,
                    ),
                    onPressed: () {
                      _focus.unfocus();
                      setState(() {
                        _allowLetters = !_allowLetters;
                        WidgetsBinding.instance
                            .addPostFrameCallback((_) => _focus.requestFocus());
                      });
                    },
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}