Flutter кой SliverAppBar + flexibleSpace with a button on edge

[Cleofas]

How to do this button in the right lower corner on the edge of SliverAppBar to move together in the zium:

I tried to make it through SliverPersistentHeaderDelegate, but I couldn't make a Zoom image on the background, so I returned SliverAppBar.

SliverAppBar(
automaticallyImplyLeading: false,                   
floating: true,
pinned: true,
snap: true,                       
stretch: true,
backgroundColor: Color(0xFFF8F9FD),
expandedHeight: getHeight(450),                     
flexibleSpace: FlexibleSpaceBar(   
titlePadding: EdgeInsets.only(left: 20),
collapseMode: CollapseMode.pin,
stretchModes: [
  StretchMode.zoomBackground,
  
],
background: Hero(
  tag: widget.card.id.toString(),
  child: Stack(
    children: [
      appBar(size),
      Positioned.fill(
        child: Align(
          alignment: Alignment.bottomRight,
          child: Padding(
            padding: const EdgeInsets.only(right: 20, top: 20),
            child: backButton(),
          ),
        ),
      )
    ],
  )
),

),

),

[kalyn]

I thought it was hard for someone to use. Yeah, I really had to go through SliverPersistentHeaderDelegate.

SliverPersistentHeader(
  delegate: LargeCustomHeader(
      child: appBar(size), //Картинка 
      btn: backButton(), //Кнопка
  )
),

Further created the caste SliverPersistentHeader

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
class LargeCustomHeader extends SliverPersistentHeaderDelegate {

LargeCustomHeader({

this.child,

this.btn

});

final Widget child,btn;

final double _navBarHeight = 56;

final double titleHeight = 10;

final double childrenHeight = 400;

@override

double get maxExtent => 400;
@override

double get minExtent => _navBarHeight;
// @override

// FloatingHeaderSnapConfiguration get snapConfiguration => FloatingHeaderSnapConfiguration() ;
@override

OverScrollHeaderStretchConfiguration get stretchConfiguration =>

OverScrollHeaderStretchConfiguration(

stretchTriggerOffset: maxExtent,

onStretchTrigger: () {
    },
  );

double get maxShrinkOffset => maxExtent - minExtent;
@override

bool shouldRebuild(SliverPersistentHeaderDelegate oldDelegate) {

//TODO: implement specific rebuild checks

return true;

}
@override

Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {

print(shrinkOffset);

return Container(

constraints: BoxConstraints.expand(),

child: Stack(

clipBehavior: Clip.none, fit: StackFit.loose,

children: [

child,

Positioned(

right: 20,

bottom: -25,

child: btn

)

],

),

);

}

}

The most important thing is adding OverScrollHeaderStretchConfiguration, he's in charge of the zum of the image and in Stack, point clipBehavior: Clip.none, as the overflow parameter was removed