How to achieve sliver in flutter when collapsing content is dynamic

Try with the silver appbar and make sure that your toolbarHeight is 0. Here I used just fixed height for a single element and the total height will be changed based on the number of elements or widgets you have.

import 'package:flutter/material.dart';

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

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

class _DynamicAppbarState extends State<DynamicAppbar> {
  //set the height fixed for each widget
  double fixedHeight = 50;

  // replace with coming elements
  List<String> items = [
    "dynamicWidget1",
    "dynamicWidget2",
    "dynamicWidget3",
    "dynamicWidget4",
  ];

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          title: Text("My App Bar"),
        ),
        body: DefaultTabController(
          length: 2,
          child: NestedScrollView(
            headerSliverBuilder:
                (BuildContext context, bool innerBoxIsScrolled) {
              return <Widget>[
                SliverAppBar(
                  expandedHeight: fixedHeight * items.length,
                  floating: false,
                  pinned: true,
                  snap: false,
                  toolbarHeight: 0,
                  flexibleSpace: FlexibleSpaceBar(
                    background: Column(
                      crossAxisAlignment: CrossAxisAlignment.center,
                      children: List<Widget>.generate(
                        items.length,
                        (index) {
                          return Container(
                            height: fixedHeight,
                            child: Center(
                                child: Text(
                              items[index],
                              textAlign: TextAlign.center,
                              style: TextStyle(
                                  fontSize: 24, fontWeight: FontWeight.bold),
                            )),
                          );
                        },
                      ),
                    ),
                  ),
                ),
                SliverPersistentHeader(
                  delegate: _SliverAppBarDelegate(
                    TabBar(
                      labelColor: Colors.black87,
                      unselectedLabelColor: Colors.grey,
                      tabs: [
                        Tab(icon: Icon(Icons.info), text: "Tab 1"),
                        Tab(icon: Icon(Icons.lightbulb_outline), text: "Tab 2"),
                      ],
                    ),
                  ),
                  pinned: true,
                ),
              ];
            },
            body: Center(
              child: Text("Sample text"),
            ),
          ),
        ),
      ),
    );
  }
}

class _SliverAppBarDelegate extends SliverPersistentHeaderDelegate {
  _SliverAppBarDelegate(this._tabBar);

  final TabBar _tabBar;

  @override
  double get minExtent => _tabBar.preferredSize.height;

  @override
  double get maxExtent => _tabBar.preferredSize.height;

  @override
  Widget build(
      BuildContext context, double shrinkOffset, bool overlapsContent) {
    return new Container(
      child: _tabBar,
    );
  }

  @override
  bool shouldRebuild(_SliverAppBarDelegate oldDelegate) {
    return false;
  }
}

Output:

Before and after scrolling

 // this field is used for getting height of widget dynamically.
 // you can set initial value as per your requirement.
 var _cardSize = Size(Get.width, 300);
  // this key is set to the widget of which we want to get size dynamically
  GlobalKey _key = GlobalKey();

 @override
  void initState() {
    super.initState();
    //IMPORTANT---- this will be called once the build() method gets 
    // executed. By then we will have the widget rendered.
    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      _getSizeOfWidget();
    });
  }

 //Get the size of the widget
  _getSizeOfWidget() {
    try {
        RenderBox _cardBox = _key.currentContext.findRenderObject();
        _cardSize = _cardBox.size;
        if (!mounted) return;
        setState(() {});
    } catch (e) {}
  }

//Set the key to the widget for which you need to set height dynamically 
//IMPORTANT -- set the height value using _cardSize
  Container(key: _key,child : //any child,height : _cardSize.height);

I hope this answers your question