Multi line text in Flex doesnt recalculate when runtime css is changed

<mx:Text id="txtDescription" selectable="false"
styleName="imageRolloverButtonTextDark" width="100%" textAlign="center"
text="{_rolloverText}"/>

<mx:Canvas id="box" width="100%" backgroundColor="Red">
    <mx:Text width="{box.width}" text="{someReallyLongString}" />
</mx:Canvas>