tachiyomi/app/src/main/res/layout/activity_reader.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:gravity="center"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <FrameLayout
        android:id="@+id/reader"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
    </FrameLayout>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="4dp"
        android:layout_gravity="bottom|left"
        android:background="@color/page_number_background"
        android:textColor="@color/black_87pc"
        android:textSize="12sp"
        android:id="@+id/page_number"/>

    <include layout="@layout/reader_menu"/>

</FrameLayout>