Flutter: Center Positioned

main.dart

import "package:flutter/material.dart";

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: "Some App",
      theme: ThemeData(
        primarySwatch: Colors.green,
      ),
      home: Scaffold(
        body: Stack(children: [
          Positioned(
            left: 100,
            top: 100,
            child: SomeChild(text: "Some child"),
          ),
          Positioned(
            left: 100,
            top: 100,
            child: Container(width: 5, height: 5, color: Colors.red.shade900),
          ),
          Positioned(
            left: 100,
            top: 150,
            child: SomeChild(text: "Some child with longer text"),
          ),
          Positioned(
            left: 100,
            top: 150,
            child: Container(width: 5, height: 5, color: Colors.red.shade900),
          ),
        ]),
      ),
    );
  }
}

class SomeChild extends StatelessWidget {
  final String text;
  const SomeChild({Key? key, required this.text}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Text(text);
  }
}