tachiyomi/app/src/main/res/layout/pre_migration_controller.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"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginTop="?attr/actionBarSize"
    android:animateLayoutChanges="true">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:clipToPadding="false"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:listitem="@layout/migration_source_item">

    </androidx.recyclerview.widget.RecyclerView>

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/fab"
        style="@style/Theme.Widget.FABFixed"
        android:clickable="true"
        android:focusable="true"
        android:src="@drawable/ic_arrow_forward_white_24dp"
        app:layout_anchor="@id/recycler"
        app:layout_anchorGravity="bottom|right|end"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:backgroundTint="?attr/colorAccent" />

</androidx.constraintlayout.widget.ConstraintLayout>