[Flutter - Android] Sending POST request with JSON body using `InAppWebView` results in 415 Unsupported Media Type

[Cyrille-Dakhlia]

• [x] I have read the Getting Started section
• [x] I have already searched for the same problem

Environment

Technology	Version
Flutter version	3.19.3
Plugin version	6.0.0
Android version
iOS version
macOS version
Xcode version
Google Chrome version

Device information:

• All Android emulators
• Samsung Galaxy A33 5G
• Samsung Galaxy Tab A8

Description

In my Flutter app, I want my logged-in user to be able to click on a button that opens an internal browser to access a server requiring an authentication, and I want to pass the user's JWT auth token in the request to avoid the user to authenticate to that server. I have read the Getting Started section I have the following configurations:

• minSdkVersion 21
• <uses-permission android:name="android.permission.INTERNET"/>

<meta-data
  android:name="flutterEmbedding"
  android:value="2" />

• I created my app in summer 2023, so I guess I do not need to worry about migrating to AndroidX (do I?)

Expected behavior: Using InAppWebView to open an internal browser, configuring the initialUrlRequest to send a POST request with a JSON content, I expect to have my request received by the server and successfully detected as having its Content-Type set to application/json.

Current behavior: When opening the InAppWebView, I get a page with a

415 Unsupported Media Type
Did not attempt to load JSON data because the request Content-Type was not 'application/json'.

The exact same code works for iOS (I get the expect web page), but not for Android (I get the 415)

Steps to reproduce

1. Have a simple button opening the preconfigured InAppWebView sending an initial POST request with a server expecting for the answer
2. Notice it works on iOS but not on Android

...
ElevatedButton(
   onPressed: () => Navigator.push(context,
      MaterialPageRoute(builder: (_) => const CustomInAppBrowser()),
   ),
   child: const Text('Open InAppWebView'),
)

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        leading: IconButton(
          icon: const Icon(Icons.close),
          onPressed: () {
            Navigator.of(context).pop();
          },
        ),
        title: const Text('InAppWebView'),
      ),
      body: InAppWebView(
        initialUrlRequest: URLRequest(
          url: WebUri('server_address_here/token'),
          method: 'POST',
          body: utf8.encode(
            jsonEncode({"token": jwtAuthTokenAsStringHere}),
          ),
          headers: {
            'Content-Type': 'application/json',
          },
        ),
        onWebViewCreated: (controller) {},
      ),
    );
  }
}