why does my MXML act differently than my AS?

I have a test program that, as far as I can tell, does the same thing using
an MXML component and the equivalent actionscript.  When I click on the
button to execute the AS version, everything is fine.  But when I click on
the button to execute the MXML version, I get a layout flicker.  It's bad
enough that if you try to click on the MXML button over and over really
fast, you'll wind up clicking the main app and the menu will go away.
Here's the code:


<?xml version="1.0" encoding="utf-8"?>
<Application
  xmlns="http://www.adobe.com/2006/mxml"
  layout="absolute"
  >
  <Script>
    <![CDATA[
      import mx.containers.GridItem;
      import mx.controls.Label;
      import mx.controls.DateField;
      private function addCategoryAS():void
      {
        var gi:GridItem;
        var gr:GridRow = new GridRow;

        gi = new GridItem;
        gr.addChild(gi);
        gi.addChild(new Label);

        addRow(gr);
      }

      private function addCategoryMXML():void
      {
        var gr:GridRow = new TestGridRow;
        gr.initialize(); // this is just to try to make it equivalent -
doesn't change flicker

        addRow(gr);
      }

      private function addRow(gr:GridRow):void
      {
        grid.addChildAt(gr, 0);

        var gi:GridItem;

        gi = gr.getChildAt(0) as GridItem;
        (gi.getChildAt(0) as Label).text = "item " + grid.numChildren + ":";

        gr.validateSize(true);
        var verticalGap:Number = grid.getStyle("verticalGap");
        popup.scrollRect = new Rectangle(0, 0, Math.max(popup.width +
gr.measuredWidth),
            popup.height + gr.measuredHeight + (isNaN(verticalGap) ? 0 :
verticalGap));
        if (popup.localToGlobal(new Point(0, 0)).y <
popup.owner.localToGlobal(new Point(0, 0)).y)
          popup.y -= gr.measuredHeight + (isNaN(verticalGap) ? 0 :
verticalGap);
      }
    ]]>
  </Script>

  <Canvas bottom="0">
    <PopUpButton id="myPopupButton" openAlways="true">
      <popUp>
        <Canvas id="popup">
          <Grid id="grid" paddingTop="4" paddingBottom="4" paddingLeft="4"
paddingRight="4"
            fontSize="12" backgroundColor="0xFFFFFF" borderStyle="outset"
borderThickness="2"
            >
            <GridRow>
              <GridItem>
                <Button label="Add AS" click="addCategoryAS()"/>
                <Button label="Add MXML" click="addCategoryMXML()"/>
              </GridItem>
            </GridRow>
          </Grid>
        </Canvas>
      </popUp>
    </PopUpButton>
  </Canvas>
</Application>

TestGridRow.mxml:
<?xml version="1.0" encoding="utf-8"?>
<GridRow xmlns="http://www.adobe.com/2006/mxml">
  <GridItem>
    <Label/>
  </GridItem>
</GridRow>


I've dug through the generated code and have not been able to find anything
that I think would account for the problem.  This is frustrating, because
what I really want to do is a more complicated MXML component to make it a
lot nicer to add rows in my addCategory function.  But I can't as long as
it's going to be so jittery.

Thanks!

--
Jason