Android WebView fullscreen on VR headset?

Question

I have an Android app which implements a WebView.

This WebView contains WebVR content.

If I view the HTML in the Quest's native web browser, the fullscreen function works. But it doesn't work in the Android WebView.

When I load the APK onto the Oculus Quest 2, I can launch it and interact with it in a window:

When I interact with the "VR" button on screen, the window goes blank.

I want it to go into fullscreen / immersive mode.

The Immersive API is deprecated.

I'm trying to use the Oculus Native Manifest but that crashes with:

java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.

I'm unsure what other information I need to provide - but here's my manifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    package="my.package.name">

    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@android:style/Theme.Black.NoTitleBar.Fullscreen"
        tools:targetApi="31"
        android:configChanges="density|keyboard|keyboardHidden|navigation|orientation|screenLayout|screenSize|uiMode"
        android:launchMode="singleTask"
        android:resizeableActivity="false">

        <meta-data android:name="com.oculus.intent.category.VR" android:value="vr_only"/>
        <meta-data android:name="com.oculus.supportedDevices" android:value="quest|quest2"/>
        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>
    <uses-sdk android:minSdkVersion="23"/>
    <uses-feature android:glEsVersion="0x00030001" />
</manifest>

And my MainActivity

package my.package.name

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.webkit.WebView

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        var webView = findViewById<WebView>(R.id.webview)
        webView.getSettings().setJavaScriptEnabled(true);
        webView.loadUrl("file:///android_asset/test.html");

    }
}