title: FAB anchored to app bar doesn't hide on scrolling up
body: <p>I have anchored FAB to AppBarLayout. However, on scrolling up the expected behaviour of the FAB is to hide when the toolbar collapses. This does not happen.</p>
  
  <p>Does this have to be handled specially?</p>
  
  <p>The layout I have defined is as below:</p>
  
  <pre><code>&lt;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"&gt;
  
  &lt;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" /&gt;
  
  &lt;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"&gt;
  
      &lt;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"&gt;
  
          &lt;!--&lt;ImageView--&gt;
          &lt;!--android:id="@+id/backdrop"--&gt;
          &lt;!--android:layout_width="match_parent"--&gt;
          &lt;!--android:layout_height="match_parent"--&gt;
          &lt;!--android:fitsSystemWindows="true"--&gt;
          &lt;!--android:scaleType="centerCrop"--&gt;
          &lt;!--app:layout_collapseMode="parallax" /&gt;--&gt;
  
          &lt;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" /&gt;
  
      &lt;/android.support.design.widget.CollapsingToolbarLayout&gt;
  
      &lt;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" /&gt;
  
  &lt;/android.support.design.widget.AppBarLayout&gt;
  
  &lt;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" /&gt;
  </code></pre>
  
  <p></p>