fixes [AppBar shape disappears on AppBar elevation change when scrolling](#145945)

### Description
This PR adds an example for complete custom app bar for the  `AppBar.shape` property.

### Preview

![Screenshot 2024-04-08 at 14 21 04](https://github.com/flutter/flutter/assets/48603081/ae3eda2b-b709-4652-9f2c-dd7b7dcfeb5c)

This PR was generated by `flutter update-packages --force-upgrade`.

…146445)

flutter/engine@cc741b5...1e88b2d

2024-04-08 [email protected] Roll Skia from 91aa93f2565d to 50b7ff0bf275 (6 revisions) (flutter/engine#51967)

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…146447)

flutter/engine@1e88b2d...ceb5fa2

2024-04-08 [email protected] Add `missing_code_block_language_in_doc_comment` lint to flutter/engine. (flutter/engine#51944)
2024-04-08 [email protected] Use the AOT snapshot built by the Dart SDK for the frontend server (flutter/engine#51943)
2024-04-08 [email protected] Roll Skia from 50b7ff0bf275 to aa31cad3cbef (2 revisions) (flutter/engine#51968)

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## Description

This PRs fixes a null pointer exception thrown when reset is called on a `DropdownButtonFormField` whose `onChange` callback is null.

## Related Issue

Fixes #146335

## Tests

Adds 1 tests.

Fixes bugs in the text selection positioning calculations so that they work even when the field is scaled. In many cases, we were simply translating things around without applying the proper localToGlobal (or vice versa) transform.

| Before | After |
| --- | --- |
| <img src="https://github.com/flutter/flutter/assets/389558/a5a45472-98c5-4cdf-b382-218971fd9404" /> | <img src="https://github.com/flutter/flutter/assets/389558/f396a1af-2546-4e38-a9d9-6c6edfa38d94" /> |

Partial fix for: #144685
It looks like there are other problems where transforms aren't applied properly. Putting a transform at the root of the application, above MaterialApp, will expose more problems.

<details>

<summary>Sample code</summary>

```dart
import 'package:flutter/material.dart';
import 'package:webview_flutter/webview_flutter.dart';

void main() => runApp(const _App());

class _App extends StatelessWidget {
  const _App();

  @OverRide
  Widget build(BuildContext context) {
    return const MaterialApp(home: _Home());
  }
}

class _Home extends StatefulWidget {
  const _Home();

  @OverRide
  State<_Home> createState() => _HomeState();
}

class _HomeState extends State<_Home> {
  final _controller = WebViewController();
  final TextEditingController textEditingController = TextEditingController(
    text: 'Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.',
  );
  final OverlayPortalController overlayPortalController = OverlayPortalController();

  @OverRide
  void initState() {
    super.initState();
    _controller
      ..setJavaScriptMode(JavaScriptMode.unrestricted)
      ..loadRequest(Uri.https('api.flutter.dev'));
  }

  @OverRide
  Widget build(BuildContext context) {
    overlayPortalController.show();
    return Scaffold(
      appBar: AppBar(
        title: const Text('Scaled WebView Tester'),
      ),
      body: Stack(
        children: <Widget>[
          Transform.scale(
            alignment: Alignment.topLeft,
            scale: 0.5,
            child: TextField(
              controller: textEditingController,
              maxLines: null,
            ),
          ),
          OverlayPortal(
            controller: overlayPortalController,
            overlayChildBuilder: (BuildContext context) {
              return Positioned(
                top: 0.0,
                left: 0.0,
                child: SizedBox(
                  height: 1000,
                  width: 1000,
                  child: Stack(
                    children: <Widget>[
                      Positioned(
                        top: 90.0,
                        left: 0.0,
                        child: Container(
                          height: 1.0,
                          width: 1000,
                          color: Colors.blue,
                        ),
                      ),
                      Positioned(
                        top: 102.0,
                        left: 0.0,
                        child: Container(
                          height: 1.0,
                          width: 1000,
                          color: Colors.blue,
                        ),
                      ),
                    ],
                  ),
                ),
              );
            },
          ),
        ],
      ),
    );
  }
}
```

</details>

…146449)

flutter/engine@ceb5fa2...269aa69

2024-04-08 [email protected] Roll Skia from aa31cad3cbef to 4281f51749b2 (1 revision) (flutter/engine#51969)

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…146463)

flutter/engine@269aa69...8701a9a

2024-04-08 [email protected] Roll Fuchsia Linux SDK from RNSwTRpc6cfMY99CC... to 82nvjI_UGVS8qYqyH... (flutter/engine#51972)
2024-04-08 [email protected] Roll Dart SDK from f7239b3c25af to 28b5735ad7dc (4 revisions) (flutter/engine#51970)
2024-04-08 [email protected] [et] Run GN before looking for targets. Default to build config targets (flutter/engine#51956)
2024-04-08 [email protected] Run local_engine.json builds in prod (flutter/engine#51931)

Also rolling transitive DEPS:
  fuchsia/sdk/core/linux-amd64 from RNSwTRpc6cfM to 82nvjI_UGVS8

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…146468)

flutter/engine@8701a9a...932c550

2024-04-08 [email protected] Remove #if SHELL_ENABLE_METAL checks in darwin code (flutter/engine#51947)

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…146475)

flutter/engine@932c550...0226114

2024-04-09 [email protected] Use Impeller geometry classes in DLMatrixClipTracker (flutter/engine#51919)
2024-04-09 [email protected] Roll Dart SDK from 28b5735ad7dc to 78174b41ab0f (1 revision) (flutter/engine#51976)

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…146477)

flutter/engine@0226114...5a824e2

2024-04-09 [email protected] [Impeller] ensure precision is matched between fragment and vertex stage for PowerVR GPU bug (flutter/engine#51974)

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fixes [Some images show "404 - page not found" in Flutter `README.md` ](#146419)

These images had their sources updated in flutter/website#10336

### Before
![Screenshot 2024-04-08 at 18 19 30](https://github.com/flutter/flutter/assets/48603081/56549762-b7c8-41a1-89db-cd45cd98b35a)

### After
![Screenshot 2024-04-08 at 18 19 23](https://github.com/flutter/flutter/assets/48603081/35d5772f-1144-4166-99cb-fded79550762)