将固定小部件放在TabBarView之上

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('TabBarView Demo'),
        ),
        body: Stack(
          children: [
            TabBarView(
              children: [
                // Tab 1 content
                Container(
                  color: Colors.red,
                ),
                // Tab 2 content
                Container(
                  color: Colors.blue,
                ),
              ],
            ),
            Positioned(
              top: 0,
              left: 0,
              right: 0,
              child: Container(
                height: 50,
                color: Colors.green,
                child: Center(
                  child: Text(
                    '固定小部件',
                    style: TextStyle(
                      color: Colors.white,
                      fontSize: 20,
                    ),
                  ),
                ),
              ),
            ),
          ],
        ),
        bottomNavigationBar: TabBar(
          tabs: [
            Tab(
              icon: Icon(Icons.tab),
            ),
            Tab(
              icon: Icon(Icons.tab),
            ),
          ],
        ),
      ),
    );
  }
}