Android – the Fab fixed to the app bar is not hidden when scrolling up

I have anchored the FAB to appbarlayout. However, when scrolling up, the expected behavior of the fab is to hide when the toolbar collapses. This will not happen

Does this have to be dealt with in particular?

The layout I define is as follows:

<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/rootLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true">

<android.support.v4.view.ViewPager
    android:id="@+id/viewpager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior" />

<android.support.design.widget.AppBarLayout
    android:id="@+id/appbar"
    android:layout_width="match_parent"
    android:layout_height="256dp"
    android:fitsSystemWindows="true"
    android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

    <android.support.design.widget.CollapsingToolbarLayout
        android:id="@+id/collapsing_toolbar"
        android:layout_width="match_parent"
        android:layout_height="206dip"
        android:background="@color/my_primary"
        android:fitsSystemWindows="true"
        app:contentScrim="?attr/colorPrimary"
        app:expandedTitleMarginBottom="20dp"
        app:expandedTitleMarginEnd="64dp"
        app:expandedTitleMarginStart="16dp"
        app:layout_scrollFlags="scroll|exitUntilCollapsed">

        <!--<ImageView-->
        <!--android:id="@+id/backdrop"-->
        <!--android:layout_width="match_parent"-->
        <!--android:layout_height="match_parent"-->
        <!--android:fitsSystemWindows="true"-->
        <!--android:scaleType="centerCrop"-->
        <!--app:layout_collapseMode="parallax" />-->

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            app:layout_collapseMode="pin"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Light" />

    </android.support.design.widget.CollapsingToolbarLayout>

    <android.support.design.widget.TabLayout
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="bottom"
        app:tabMode="scrollable" />

</android.support.design.widget.AppBarLayout>

<android.support.design.widget.FloatingActionButton
    android:id="@+id/fab"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_margin="16dp"
    android:clickable="true"
    android:src="@drawable/ic_stop_white_24dp"
    app:layout_anchor="@id/appbar"
    app:layout_anchorGravity="bottom|right|end" />

resolvent:

Maybe set the scroll listener to Scrollview and hide the Fab as needed