FadeForwardsPageTransitionsBuilder flickers for AppBars with dark background

### Steps to reproduce

Using `FadeForwardsPageTransitionsBuilder` (e. g. in Android or by setting it by hand like in the example).

1. Create two screens each containing a Scaffold with an AppBar with a dark background.
2. Navigate from the first screen to the second and back.

### Expected results

The AppBar should keep its dark color while navigating.

### Actual results

The AppBar flickers to a bright color.

### Code sample

Code sample

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

/// Based on https://api.flutter.dev/flutter/material/FadeForwardsPageTransitionsBuilder-class.html
/// changed AppBar backgroundColor to black and removed unnecessary parts

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

class PageTransitionsThemeApp extends StatelessWidget {
const PageTransitionsThemeApp({super.key});

@override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
theme: ThemeData(
pageTransitionsTheme: PageTransitionsTheme(
builders: Map.fromIterable(
TargetPlatform.values,
value: (_) => const FadeForwardsPageTransitionsBuilder(),
),
),
),
home: const HomePage(),
);
}
}

class HomePage extends StatelessWidget {
const HomePage({super.key});

@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
foregroundColor: Colors.white,
backgroundColor: Colors.black,
),
body: Center(
child: FilledButton(
onPressed: () {
Navigator.of(context).push(MaterialPageRoute(builder: (context) => SecondPage()));
},
child: Text("Navigate"),
),
),
);
}
}

class SecondPage extends StatelessWidget {
const SecondPage({super.key});

@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
foregroundColor: Colors.white,
backgroundColor: Colors.black,
),
body: Center(child: Text("Second Screen")),
);
}
}

```

### Screenshots or Video

Video demonstration

https://github.com/user-attachments/assets/f72b8fa9-d9cc-4612-b135-dc926ba894f7

### Logs

_No response_

### Flutter Doctor output

Also reproducible on DartPad

Doctor output

```console
[✓] Flutter (Channel stable, 3.38.4, on macOS 26.1 25B78 darwin-arm64, locale en-DE) [463ms]
• Flutter version 3.38.4 on channel stable at /Users/olischne/fvm/versions/stable
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision 66dd93f9a2 (5 days ago), 2025-12-03 14:56:10 -0800
• Engine revision a5cb96369e
• Dart version 3.10.3
• DevTools version 2.51.1
• Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations, enable-native-assets, omit-legacy-version-file,
enable-lldb-debugging

[✓] Android toolchain - develop for Android devices (Android SDK version 36.1.0) [1,663ms]
• Android SDK at /Users/olischne/Library/Android/sdk
• Emulator version 36.3.10.0 (build_id 14472402) (CL:N/A)
• Platform android-36, build-tools 36.1.0
• Java binary at: /Users/olischne/Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
This is the JDK bundled with the latest Android Studio installation on this machine.
To manually set the JDK path, use: `flutter config --jdk-dir="path/to/jdk"`.
• Java version OpenJDK Runtime Environment (build 21.0.8+-14196175-b1038.72)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 26.0.1) [1,391ms]
• Xcode at /Applications/Xcode.app/Contents/Developer
• Build 17A400
• CocoaPods version 1.16.2

[✓] Chrome - develop for the web [6ms]
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Connected device (4 available) [9.3s]
• sdk gphone64 arm64 (mobile) • emulator-5554 • android-arm64 • Android 16 (API 36) (emulator)
• iPhone 17 Pro (mobile) • 87B49BC9-D1F0-4E16-8744-53847B35AA7D • ios • com.apple.CoreSimulator.SimRuntime.iOS-26-0 (simulator)
• macOS (desktop) • macos • darwin-arm64 • macOS 26.1 25B78 darwin-arm64
• Chrome (web) • chrome • web-javascript • Google Chrome 143.0.7499.40

[✓] Network resources [1,751ms]
• All expected network resources are available.

• No issues found!
```