Flutter Fixed Button in CustomScrollView

You can use floatingActionButton and floatingActionButtonLocation

exam:

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new MyHomePage(),
    );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        floatingActionButton: FloatingActionButton.extended(
          onPressed: () {},
          isExtended: true,
          materialTapTargetSize: MaterialTapTargetSize.shrinkWrap,
          icon: Icon(Icons.supervised_user_circle),
          label: Text('Fixed Button'),
        ),
        floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
        body: CustomScrollView(
          slivers: <Widget>[
            const SliverAppBar(
              pinned: true,
              expandedHeight: 250.0,
              flexibleSpace: FlexibleSpaceBar(
                title: Text('Demo'),
              ),
            ),
            SliverGrid(
              gridDelegate: SliverGridDelegateWithMaxCrossAxisExtent(
                maxCrossAxisExtent: 200.0,
                mainAxisSpacing: 10.0,
                crossAxisSpacing: 10.0,
                childAspectRatio: 4.0,
              ),
              delegate: SliverChildBuilderDelegate(
                (BuildContext context, int index) {
                  return Container(
                    alignment: Alignment.center,
                    color: Colors.teal[100 * (index % 9)],
                    child: Text('grid item $index'),
                  );
                },
                childCount: 20,
              ),
            ),
            SliverFixedExtentList(
              itemExtent: 50.0,
              delegate: SliverChildBuilderDelegate(
                (BuildContext context, int index) {
                  return Container(
                    alignment: Alignment.center,
                    color: Colors.lightBlue[100 * (index % 9)],
                    child: Text('list item $index'),
                  );
                },
              ),
            ),
          ],
        ));
  }
}

One Way of Doing it - using - BottomNavigationBar

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: <Widget>\[
          SliverAppBar(
            title: Text('Sliver App Bar'),
            expandedHeight: 125.0,
          ),
          SliverList(
              delegate: SliverChildBuilderDelegate((context, int) {
            return Text('Boo');
          }, childCount: 65)),
          SliverFillRemaining(
            child: Text('Foo Text'),
          ),
        \],
      ),
      bottomNavigationBar: Padding(
        padding: EdgeInsets.all(8.0),
        child: RaisedButton(
          onPressed: () {},
          color: Colors.blue,
          textColor: Colors.white,
          child: Text('Fixed Button'),
        ),
      ),
    );
  }][1]][1]

OutPut: