AndroidStudio random whitespace at top of app

Question

I'm very new to Android Studio and thre's this whitespace that I haven't defined at the top of my app when I launch it, but not in the preview screen in Android Studio:

I have no idea where it's coming from, as the top of my definition XML doesn't define this whitespace:

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/scrollContainer"
    android:layout_width="match_parent"
    android:layout_height="match_parent">


    <LinearLayout
        android:id="@+id/containerLayout"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">

Below this is just two more LinearLayouts inside the one shown above. I've tried just adding text and the whitespace still appears. I also haven't added anything in code for this to appear.

Any help / ideas would be appreciated!

Edit: Below is the activity_main XML:

    <?xml version="1.0" encoding="utf-8"?>
    <androidx.constraintlayout.widget.ConstraintLayout 
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:paddingTop="?attr/actionBarSize">

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/nav_view"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/bottom_nav_menu" />

    <fragment
        android:id="@+id/nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toTopOf="@id/nav_view"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/mobile_navigation" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 1

In activity_main.xml, try removing the line:

android:paddingTop="?attr/actionBarSize"

This adds padding around the ConstraintLayout content equal to the height of the action bar, which is not needed in this case.