instant back transition minimal example

main.dart

import 'package:flutter/material.dart';

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const Home(),
    );
  }
}

class Home extends StatelessWidget {
  const Home({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: TextButton(
        onPressed: () => Navigator.of(context).push(
          PageRouteBuilder(
            transitionDuration: Duration.zero,
            reverseTransitionDuration: Duration.zero,
            pageBuilder: (context, animation, secondaryAnimation) =>
                const Page(),
          ),
        ),
        child: const Text('go to page'),
      ),
    );
  }
}

class Page extends StatelessWidget {
  const Page({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: const Text('page'),
    );
  }
}