Flex 4: Build a Group with a background

I'm trying to build a simple component extending spark.components.Group to have a background color, more specifically a spark.primitives.Rect component stretched to fill the background.

This is what I've come up with thus far:

<s:Group xmlns:fx="http://ns.adobe.com/mxml/2009" xmlns:s="library://ns.adobe.com/flex/spark" xmlns:mx="library://ns.adobe.com/flex/halo"
    <fx:Metadata>
        [DefaultProperty(name="content")]
    </fx:Metadata>

    <s:Rect id="background" width="100%" height="100%">
        <s:fill>
            <s:SolidColor color="#990000"/>
        </s:fill>
    </s:Rect>

    <s:Group id="container"/>

    <s:filters>
        <!-- For good measure ;) -->
        <s:DropShadowFilter color="#000000" strength="0.4" blurX="5" blurY="5" distance="2" angle="90"/>
    </s:filters>

    <fx:Script>
        <![CDATA[
            public function set content(value:Array):void {
                this.container.mxmlContent = value;
            }
        ]]>
    </fx:Script>
</s:Group>

Ok, so the logic here basically makes sense, right? All children declared in MXML go to the Group called "container". That is working just fine. However, when I run the example below, the layout is completely fubar.

<s:VGroup>
    <!-- This is the component described above -->
    <components:MessageContainer id="component" width="100" height="100"/>
    <mx:Slider/>
    <mx:Slider/>
    <mx:ColorPicker/>
</s:VGroup>

This is what it looks like:

Is there something I'm missing here? Maybe a method I need to override?

Weird. I just resorted to making a Skin implementation and applying it to a SkinnableContainer. I was compiling using the Beta 2 release of Flex 4 as well. Very strange indeed.