Repository: flutter
Issue: Visual Anomaly: Shadow Present on [SearchBar]
Labels: framework,f: material design,has reproducible steps,P3,team-design,triaged-design,found in release: 3.16,found in release: 3.18

### Steps to reproduce

Flutter 3.16.4 • channel stable • https://github.com/flutter/flutter.git
Framework • revision 2e9cb0aa71 (4 days ago) • 2023-12-11 14:35:13 -0700
Engine • revision 54a7145303
Tools • Dart 3.2.3 • DevTools 2.28.4
I only use the example provided by the documentation, indeed with that example I can see the shadow
[https://api.flutter.dev/flutter/material/SearchBar-class.html](https://api.flutter.dev/flutter/material/SearchBar-class.html)

### Expected results

The component without the shadow
<img width="411" alt="Screenshot 2023-12-16 at 12 46 38 a m" src="https://github.com/flutter/flutter/assets/22499775/ecf90815-b62a-4a0e-b1d8-5137cb19f606">


### Actual results

The component with a shadow
<img width="381" alt="Screenshot 2023-12-16 at 12 45 57 a m" src="https://github.com/flutter/flutter/assets/22499775/ff9aa15f-4342-46bf-993c-89b0b58202b2">



### Code sample

<details open><summary>Code sample</summary>

```dart
import 'package:flutter/material.dart';

/// Flutter code sample for [SearchBar].

void main() => runApp(const SearchBarApp());

class SearchBarApp extends StatefulWidget {
  const SearchBarApp({super.key});

  @override
  State<SearchBarApp> createState() => _SearchBarAppState();
}

class _SearchBarAppState extends State<SearchBarApp> {
  bool isDark = false;

  @override
  Widget build(BuildContext context) {
    final ThemeData themeData = ThemeData(
        useMaterial3: true,
        brightness: isDark ? Brightness.dark : Brightness.light);

    return MaterialApp(
      theme: themeData,
      home: Scaffold(
        appBar: AppBar(title: const Text('Search Bar Sample')),
        body: Padding(
          padding: const EdgeInsets.all(8.0),
          child: SearchAnchor(
              builder: (BuildContext context, SearchController controller) {
            return SearchBar(
              controller: controller,
              padding: const MaterialStatePropertyAll<EdgeInsets>(
                  EdgeInsets.symmetric(horizontal: 16.0)),
              onTap: () {
                controller.openView();
              },
              onChanged: (_) {
                controller.openView();
              },
              leading: const Icon(Icons.search),
              trailing: <Widget>[
                Tooltip(
                  message: 'Change brightness mode',
                  child: IconButton(
                    isSelected: isDark,
                    onPressed: () {
                      setState(() {
                        isDark = !isDark;
                      });
                    },
                    icon: const Icon(Icons.wb_sunny_outlined),
                    selectedIcon: const Icon(Icons.brightness_2_outlined),
                  ),
                )
              ],
            );
          }, suggestionsBuilder:
                  (BuildContext context, SearchController controller) {
            return List<ListTile>.generate(5, (int index) {
              final String item = 'item $index';
              return ListTile(
                title: Text(item),
                onTap: () {
                  setState(() {
                    controller.closeView(item);
                  });
                },
              );
            });
          }),
        ),
      ),
    );
  }
}

```

</details>