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

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

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

    <ProgressBar
        android:id="@+id/please_wait"
        style="?android:attr/progressBarStyleLarge"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center"/>

    <TextView
        android:id="@+id/page_number"
        style="@style/TextAppearance.Regular.Caption"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|start"
        android:background="?android:attr/colorBackground"
        android:padding="4dp"/>

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

</FrameLayout>