How do I get default navigation bar in this hello world?

Question

MainActivity.java

package com.example.test;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;
import android.view.ViewGroup.LayoutParams;
import android.view.Gravity;
import android.widget.LinearLayout;
import android.view.WindowManager;
import android.view.View;
import android.view.Window;

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView txt = new TextView(this);
        txt.setLayoutParams(new LayoutParams(
                                LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT
                            ));
        txt.setText("Hello, world!");

        LinearLayout lay = new LinearLayout(this);
        lay.setLayoutParams(new LayoutParams(
                                LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT
                            ));
        lay.addView(txt);
        setContentView(lay);
    }
}

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.example.test"
          versionCode="1"
          versionName="0.1">
    <uses-sdk android:minSdkVersion="21"/>
    <application android:label="test">
        <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>
</manifest>

And I get

Where are the three nagivation buttons ? Why is there a fullscreen icon at bottom right ? Is this navigation bar or something else ? How do I get the app window sandwiched between status bar and navigation bar ?

I tested this on Xiaomi Redmi Note 5 Pro (APILEVEL=28).

Answer 1

After much head banging I figure out that app runs in some "compatibility mode" if targetSdkVersion is too low (may be relative to device apilevel). targetSdkVersion >=24 fixed it for me. Shitty documentation Android :/