Why can't I nest a SplitPane inside a BoxPane?

[Bill van Melle]

I'm feeling stupid again.  In this example, which nests a SplitPane inside a
BoxPane, the SplitPane doesn't display at all, but I do see the other
element in the BoxPane.  If I elevate the SplitPane to be the child of
Border, getting rid of the BoxPane and its other child, it displays fine.
 What am I missing?

<my:TestWindow maximized="true"
  xmlns:bxml="http://pivot.apache.org/bxml";
  xmlns:my="com.fxpal.test"
  xmlns="org.apache.pivot.wtk">
  <Border styles="{color:10, thickness:5, cornerRadii:10}">
    <BoxPane bxml:id="paneMain" styles="{padding:4, verticalAlignment:'top',
fill:true}"
      orientation="vertical">
      <BoxPane styles="{padding:4, horizontalAlignment:'center',
backgroundColor:'yellow'}">
        <Label text="Header pane" styles="{horizontalAlignment:'center',
font:{size:20}}" />
      </BoxPane>
      <SplitPane bxml:id="paneSplit" orientation="vertical">
        <top>
          <Border styles="{backgroundColor:'pink'}">
            <Label text="top pane" />
          </Border>
        </top>
        <bottom>
          <Border styles="{backgroundColor:'green'}">
            <Label text="bottom pane" />
          </Border>
        </bottom>
      </SplitPane>
    </BoxPane>
  </Border>
</my:TestWindow>