selection issue

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,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  var controller = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(''),
      ),
      body: Builder(
        builder: (context) {
          return TextField(
            controller: controller,
            keyboardType: TextInputType.number,
            onChanged: (value) {
              debugPrint('onChange called with: $value');
              var validatedValue = value.replaceAll(RegExp('[^0-9]'), '');
              controller.text = validatedValue;
              controller.selection = TextSelection.collapsed(offset: validatedValue.length);
              debugPrint('cursor position set at: ${validatedValue.length}');
            },
          );
        },
      ),
    );
  }
}