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

<FrameLayout 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:gravity="center">

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

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

        <ProgressBar
            android:id="@+id/please_wait"
            android:layout_width="56dp"
            android:layout_height="56dp"
            android:layout_gravity="center"
            android:visibility="gone"
            tools:visibility="visible" />

        <eu.kanade.tachiyomi.ui.reader.PageIndicatorTextView
            android:id="@+id/page_number"
            style="@style/TextAppearance.Regular.Caption"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="bottom|center_horizontal"
            android:padding="4dp"
            android:textStyle="bold" />

    </FrameLayout>

    <FrameLayout
        android:id="@+id/reader_menu"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fitsSystemWindows="true"
        android:theme="?attr/actionBarTheme"
        android:visibility="invisible"
        tools:visibility="visible">

        <androidx.appcompat.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?colorPrimary" />

        <LinearLayout
            android:id="@+id/reader_menu_bottom"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:layout_gravity="bottom"
            android:background="?colorPrimary"
            android:descendantFocusability="blocksDescendants"
            android:gravity="center"
            android:orientation="horizontal">

            <ImageButton
                android:id="@+id/left_chapter"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:background="?selectableItemBackgroundBorderless"
                android:contentDescription="@string/action_previous_chapter"
                android:padding="@dimen/material_layout_keylines_screen_edge_margin"
                app:srcCompat="@drawable/ic_skip_previous_white_24dp" />

            <TextView
                android:id="@+id/left_page_text"
                android:layout_width="32dp"
                android:layout_height="match_parent"
                android:gravity="center"
                android:textSize="15sp"
                tools:text="1" />

            <!--
                Wonky way of setting height due to issues with horizontally centering the thumb in Android 5.
                See https://stackoverflow.com/questions/15701767/android-thumb-is-not-centered-in-seekbar
            -->
            <eu.kanade.tachiyomi.ui.reader.ReaderSeekBar
                android:id="@+id/page_seekbar"
                android:layout_width="0dp"
                android:layout_height="wrap_content"
                android:maxHeight="?attr/actionBarSize"
                android:minHeight="?attr/actionBarSize"
                android:layout_weight="1" />

            <TextView
                android:id="@+id/right_page_text"
                android:layout_width="32dp"
                android:layout_height="match_parent"
                android:gravity="center"
                android:textSize="15sp"
                tools:text="15" />

            <ImageButton
                android:id="@+id/right_chapter"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:background="?selectableItemBackgroundBorderless"
                android:contentDescription="@string/action_next_chapter"
                android:padding="@dimen/material_layout_keylines_screen_edge_margin"
                app:srcCompat="@drawable/ic_skip_next_white_24dp" />

        </LinearLayout>

    </FrameLayout>

    <View
        android:id="@+id/brightness_overlay"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:visibility="gone" />

    <eu.kanade.tachiyomi.ui.reader.ReaderColorFilterView
        android:id="@+id/color_overlay"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:visibility="gone" />

</FrameLayout>