title: Android ImageButton in Material Design shows no shadow
body: <p>I can't display the material design shadow under any component of my layout, I can't understand why.</p>
  
  <p>Android minimum sdk is 21, <code>&lt;style name="AppTheme" parent="android:Theme.Material.Light"&gt;</code>, and if for example I add a FabButton its shadow works properly according to its elevation parameter. But not for my ImageButton or any other component.</p>
  
  <p>Its layout is:</p>
  
  <pre><code>&lt;ImageButton
      android:id="@+id/btnMain"
      android:layout_width="match_parent"
      android:layout_height="100dp"
      android:layout_alignParentStart="true"
      android:layout_below="@+id/txtTitle"
      android:adjustViewBounds="true"
      android:background="@null"
      android:contentDescription="@string/btnMain_description"
      android:elevation="10dp"
      android:padding="10dp"
      android:scaleType="centerInside"
      android:src="@drawable/btn_yellow"
      app:srcCompat="@android:color/holo_red_light"
      android:layout_marginLeft="10dp"
      android:layout_marginRight="10dp"
      android:layout_marginTop="10dp" /&gt;
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/CTZ1g.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CTZ1g.png" alt="The result with no shadow"></a></p>
  
  <p>I also already tried to set a background different than @null, nothing changes, no shadows. What's going on? Which way my I have my elevation shadow on layout components?</p>