Repository: flutter
Issue: SliverReorderableList rebuilds all visible elements when dragging
Labels: framework,f: scrolling,has reproducible steps,P2,found in release: 2.10,found in release: 2.13,team-framework,triaged-framework

I've noticed some performance issues with a reorderable list I have in my app, which I've attributed to a huge number of rebuilds being triggered when dragging items around.

It looks like `SliverReorderableList` rebuilds all visible children every frame that there is a drag update. Is this necessary? It seems like at the most, rebuilds should only be necessary for the items being reordered. Instead, every on-screen child gets rebuilt if another item in the list is dragged a few pixels.

## Details

1. Run the example app below in debug mode
2. Vertically drag an item around in the list -- only slightly, without dragging so far that a reorder is triggered
3. Notice the printed logs indicating all the items being rebuilt

There isn't a noticeable performance issue in this demo, but with such frequent rebuilds, it's easy to imagine how this can cause issues for reorderable elements with more expensive build methods.

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key}) : super(key: key);

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final List<int> _items = List<int>.generate(50, (int index) => index);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: CustomScrollView(
        slivers: [
          SliverReorderableList(
            itemCount: _items.length,
            itemBuilder: (context, index) {
              print(index);

              return ReorderableDragStartListener(
                index: index,
                key: Key('$index'),
                child: Container(
                  color: Colors.white,
                  padding: const EdgeInsets.all(40.0),
                  child: Text(_items[index].toString()),
                ),
              );
            },
            onReorder: (int oldIndex, int newIndex) {
              setState(() {
                if (oldIndex < newIndex) {
                  newIndex -= 1;
                }

                final int item = _items.removeAt(oldIndex);
                _items.insert(newIndex, item);
              });
            },
          ),
        ],
      ),
    );
  }
}
```

I've tested the issue on both web and Android, and the issue occurs on both platforms. It occurs on the latest stable and master branches.

## Logs

<details>
<summary>Logs</summary>

```
[✓] Flutter (Channel stable, 2.10.0, on macOS 12.2.1 21D62 darwin-x64, locale en-US)
    • Flutter version 2.10.0 at /Users/bryson/flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 5f105a6ca7 (10 weeks ago), 2022-02-01 14:15:42 -0800
    • Engine revision 776efd2034
    • Dart version 2.16.0
    • DevTools version 2.9.2

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
    • Android SDK at /Users/bryson/Library/Android/sdk
    • Platform android-31, build-tools 29.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 13.3)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • CocoaPods version 1.11.2

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 4.1)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6915495)

[✓] VS Code (version 1.66.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.38.1

[✓] Connected device (4 available)
    • Pixel 6 Pro (mobile) • 1A191FDEE007MZ                           • android-arm64  • Android 12 (API 32)
    • iPhone X (mobile)    • 8ca2a91e9a7b3818f0cd37d0aa1c08b65921f2a9 • ios            • iOS 15.3.1 19D52
    • macOS (desktop)      • macos                                    • darwin-x64     • macOS 12.2.1 21D62 darwin-x64
    • Chrome (web)         • chrome                                   • web-javascript • Google Chrome 100.0.4896.75

[✓] HTTP Host Availability
    • All required HTTP hosts are available

• No issues found!
```

</details>