CollapsingToolbarLayout без тени в расширенном состоянии

CollapsingToolbarLayout из appcompat показывает тень в сложенном состоянии, но при расширении (или разворачивании в процессе) тени исчезают

Мой пример кода https://github.com/NaikSoftware/CollapsingToolbarWithImageAndTabs/tree/master/app

Макет

<?xml version="1.0" encoding="utf-8"?>

<android.support.design.widget.CoordinatorLayout 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:id="@+id/main_content"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="ua.naiksoftware.hidetabs.MainActivity">

<android.support.design.widget.AppBarLayout
    android:id="@+id/appbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.design.widget.CollapsingToolbarLayout
        android:layout_width="match_parent"
        android:layout_height="@dimen/toolbar_plus_tabs"
        app:contentScrim="@android:color/transparent"
        app:layout_scrollFlags="scroll|exitUntilCollapsed|snap|enterAlways"
        app:titleEnabled="false"
        app:toolbarId="@+id/toolbar_wrapper">

        <ImageView
            android:id="@+id/appbar_background"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:scaleType="centerCrop"
            android:src="@drawable/header_back"
            app:layout_collapseMode="parallax"/>

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_gravity="top"
            android:background="@android:color/transparent"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar_wrapper"
            android:layout_width="match_parent"
            android:layout_gravity="bottom"
            android:background="@android:color/transparent"
            android:layout_height="wrap_content"
            android:minHeight="@dimen/tab_layout_height">

            <android.support.design.widget.TabLayout
                android:id="@+id/tabs"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:background="@android:color/transparent"
                app:tabGravity="fill"
                app:tabIndicatorColor="@android:color/white"
                app:tabMode="scrollable"
                app:tabSelectedTextColor="@android:color/white" />
        </android.support.v7.widget.Toolbar>

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

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

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

<android.support.design.widget.FloatingActionButton
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:src="@drawable/ic_add_black_24dp"
    android:layout_margin="@dimen/fab_margin"
    android:tint="@android:color/white"
    android:layout_gravity="bottom|end"
    app:elevation="@dimen/fab_elevation"
    app:layout_behavior="ua.naiksoftware.hidetabs.FabSlidingBehavior"/>

Рухнул

Расширенные

Ответы

<android.support.design.widget.AppBarLayout
    android:id="@+id/appbar"
    android:layout_width="match_parent"
    android:layout_height="320dp"
    android:stateListAnimator="@drawable/appbar_always_elevated"
    android:fitsSystemWindows="true">

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <objectAnimator android:propertyName="elevation"
            android:valueTo="8dp"
            android:valueType="floatType"
            android:duration="1"/>
    </item>
</selector>

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:tools="http://schemas.android.com/tools"
    xmlns:android="http://schemas.android.com/apk/res/android"
    tools:ignore="PrivateResource">
    <item>
        <objectAnimator
            android:propertyName="elevation"
            android:valueTo="@dimen/design_appbar_elevation"
            android:valueType="floatType" />
    </item>
</selector>