[Android Q&A, Help & Troubleshooting] fossette:
Hello! I generated a very simple 2 activities app using Google's Android Studio, and the target is an Android 8.1 phone. The problem is that the main activity doesn't show the edit box and buttons (that are obviously visible in the Design page). If I start my login activity (in the onCreate() function of the main activity), the login activity UI is visible and the main activity UI is still not visible. If I single step debug the app, the main activity UI seems present (as no error is encountered). They are just invisible. I compared the XML of both activities, and couldn't find a different setting that would fix this problem.

PS: I could't find a solution in Google search either.

PS: I added a space to break the URL links below.

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http: //schemas.android.com/apk/res/android"
package="com.example.test2">

<application
android:allowBackup="true"
android:allowTaskReparenting="false"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="false"
android:theme="@style/AppTheme">
<activity
android:alwaysRetainTaskState="false"
android:label="@string/app_name"
android:launchMode="singleInstance"
android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:alwaysRetainTaskState="false"
android:label="@string/app_name"
android:name=".LoginActivity"
android:parentActivityName=".MainActivity">
<meta-data
android:name="android.support.PARENT_ACTIVITY"
android:value="com.example.test2.MainActivity" />
</activity>
</application>
</manifest>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http: //schemas.android.com/apk/res/android"
xmlns:app="http: //schemas.android.com/apk/res-auto"
xmlns:tools="http: //schemas.android.com/tools"
android:id="@+id/containerMain"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingStart="16dp"
android:paddingTop="16dp"
android:paddingEnd="16dp"
android:paddingBottom="16dp"
android:visibility="visible"
tools:context=".MainActivity">

<android.support.design.widget.TextInputLayout
android:id="@+id/textInputLayout"
android:layout_width="0dp"
android:layout_height="46dp"
android:layout_marginStart="16dp"
android:layout_marginTop="24dp"
android:layout_marginEnd="8dp"
android:visibility="visible"
app:layout_constraintEnd_toEndOf="parent">

<android.support.design.widget.TextInputEditText
android:id="@+id/textName"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="@string/TextNameHint"
android:visibility="visible" />
</android.support.design.widget.TextInputLayout>

<Button
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginTop="24dp"
android:layout_marginEnd="8dp"
android:clickable="true"
android:enabled="true"
android:focusable="true"
android:longClickable="false"
android:onClick="buttonClick"
android:text="@string/Button"
android:visibility="visible"
app:layout_constraintEnd_toEndOf="parent" />
</android.support.constraint.ConstraintLayout>

MainActivity.java

package com.example.test2;

import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.ToggleButton;

public class MainActivity extends AppCompatActivity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}

Thanks in advance!