In the pageview item need to add border

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatefulWidget {
  // This widget is the root of your application.
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(),
    );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {
  double _deviceHeight = 0;
  double _deviceWidth = 0;
  List<MaterialColor> _colorList = List();

  @override
  void initState() {
    super.initState();
    for (int i = 0; i < 10; i++) {
      if (i % 2 == 0) {
        _colorList.add(Colors.red);
      } else {
        _colorList.add(Colors.blue);
      }
    }
  }

  @override
  Widget build(BuildContext context) {
    _deviceHeight = MediaQuery.of(context).size.height;
    _deviceWidth = MediaQuery.of(context).size.width;

    return Scaffold(
        body: Container(
      child: Stack(children: [
        PageView.builder(
            itemCount: _colorList.length,
            itemBuilder: (BuildContext context, int index) {
              return Container(
                color: _colorList[index],
              );
            }),
        Positioned(
          top: 0.0,
          left: -3.0,
          right: 0.0,
          child: PreferredSize(
            preferredSize: Size.fromHeight(100.0),
            child: Container(
              margin: EdgeInsets.fromLTRB(5.0, 10.0, 0.0, 10.0),
              child: AppBar(
                backgroundColor: Colors.transparent,
                elevation: 0,
                centerTitle: false,
                titleSpacing: 0.0,
                automaticallyImplyLeading: false,
                title: Text(
                  "Welcome to flutter ",
                  style: TextStyle(color: Colors.white),
                  textAlign: TextAlign.center,
                ),
              ),
            ),
          ),
        ),
        Positioned(
            top: _deviceHeight / 8,
            left: -3.0,
            right: 0.0,
            child: Center(
              child: Text(
                "Hi",
                style: TextStyle(color: Colors.white, fontSize: 60),
                textAlign: TextAlign.center,
              ),
            )),
        Positioned(
            top: _deviceHeight / 1.5,
            left: 0.0,
            right: 0.0,
            child: Text(
              "Bottom",
              style: TextStyle(color: Colors.white, fontSize: 60),
              textAlign: TextAlign.center,
            )),
      ]),
    ));
  }

  @override
  void dispose() {
    super.dispose();
  }
}