如何在CoordinatorLayout中使用behavior_overlayTop？

Question

一直试图在CoordinatorLayout孩子中使用behavior_overlayTop，如中所述 Overlap scrolling view with AppBarLayout

<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/main_content"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <android.support.design.widget.AppBarLayout
        android:id="@+id/app_bar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <include
            android:id="@+id/tool_bar"
            layout="@layout/tool_bar" />

        <android.support.design.widget.TabLayout
            android:id="@+id/tab_layout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:tabMaxWidth="0dp"
            app:tabGravity="fill"
            app:tabMode="fixed"
            android:background="@color/tab_layout_color"
            app:layout_scrollFlags="scroll|enterAlways|snap"/>

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

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"
        app:behavior_overlapTop="64dp">

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

这是tool_bar布局：

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="@drawable/app_bar_background"
    android:minHeight="?attr/actionBarSize"
    app:contentInsetLeft="0dp"
    app:contentInsetStart="0dp"
    app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
    app:layout_scrollFlags="scroll|enterAlways|snap">

</android.support.v7.widget.Toolbar>

但ViewPager始终显示在AppBarLayout下方且与AppBarLayout不重叠。有人可以解释behavior_overlayTop的正确用法吗？