How do I build a Scroller from scratch in Actionscript?

Question

I have a hierarchy of objects in my Actionscript.  I use Actionscript because all objects are dynamic, built at run-time on the basis of data from the server.
     Scroller
          Group
               SxText (my class) is a subclass of RichEditableText which implements IViewPort
Typical code is:
public class SxText extends RichEditableText
            sxText = new sxText(....);
            scroller = new Scroller();
            scroller.setStyle("horizontalScrollPolicy","off");
            scroller.setStyle("verticalScrollPolicy","on");  
            group = new Group();
            group.addElement(sxText);
            scroller.viewport = group;
It looks great and I can control location and size, as well as detect the events that I'm interested in.
But, the scroll bar thumb fills the vertical space of the bar.
I've tried many methods to enable or to set viewport content size, but the following code has no effect.
            scrollerSkin = ScrollerSkin(scroller.skin);
            scrollBar = scrollerSkin.verticalScrollBar;
            vbarSkin = VScrollBarSkin(scrollBar.skin);
            thumb = vbarSkin.thumb;
            vbarSkin.addEventListener(MouseEvent.CLICK,scrollBarEvent);
            vbarSkin.setContentSize(sxText.width,sxText.height);
I'm guessing that I'm missing the one method that will do it.  Thanks.

Answer 1

After posting, I kept trying.  This works, making the thumb usable.
This code is called in an event handler of the scroller for UPDATE_COMPLETE
            group.setContentSize( good values for dx and dy );
Thanks for listening.