title: Android - CollapsingToolbarLayout elevation shadow disappears when setting exitUntilCollapsed
body: <p>I'm using a custom AppBarLayout because I want a custom shape for my toolbar with an elevation shadow.   The background is set on the AppBarLayout as a custom Drawable and I use the following code to to get the shadow to render on the toolbar.</p>
  
  <pre><code>setClipToOutline(true);
  setOutlineProvider(ViewOutlineProvider.BACKGROUND);
  </code></pre>
  
  <p>All is good, the shadow renders. However, my CollapsingToolbarLayout also contains some tabs and when I collapse the toolbar I want my tabs to stay visible. Currently when I scroll the entire toolbar disappears including the tabs ands it's only when I set exitUntilCollapse on the CollapsingToolbarLayout that the tabs remain after scrolling.  The catch is when I set this flag my elevation shadow stops rendering!  Why could this be? Is there a way around it, is there another way to stop my tabs disappearing?  </p>
  
  <p>I've included some of my relevant layout code below:</p>
  
  <pre><code>    &lt;com.ss.views.CustomBackgroundAppBarLayout
          android:id="@+id/main.appbar"
          android:layout_width="match_parent"
          android:layout_height="350dp"
          android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
          app:layout_behavior="com.ss.views.FlingBehaviour"
          &gt;
  
  
        &lt;android.support.design.widget.CollapsingToolbarLayout
            android:id="@+id/main.collapsing"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:fitsSystemWindows="true"
            app:expandedTitleMarginBottom="0dp"
            app:expandedTitleMarginEnd="0dp"
            app:expandedTitleMarginStart="0dp"
            app:expandedTitleMarginTop="0dp"
            app:layout_scrollFlags="scroll|snap|exitUntilCollapsed"
            app:titleEnabled="false"
            &gt;
  
          &lt;include
              android:id="@+id/tab_layout"
              layout="@layout/include_tab_layout"
              /&gt;
  
          &lt;android.support.v7.widget.Toolbar
              android:id="@+id/main.toolbar"
              android:minHeight="100dp"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              app:layout_collapseMode="parallax"
              app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
              /&gt;
  
          &lt;include
            android:id="@+id/user_dash"
            app:layout_collapseMode="parallax"
            layout="@layout/include_toolbar_user_dash"/&gt;
  
        &lt;/android.support.design.widget.CollapsingToolbarLayout&gt;
      &lt;/com.ss.views.CustomBackgroundAppBarLayout&gt;
  </code></pre>