Callout arrow position not updated

Lets define stuff:
var callout:spark.components.Callout = ...;
var calloutAnchor:Group = new Group();
calloutAnchor.width = 0; calloutAnchor.height = 0;

Steps to reproduce:
1. callout.open(calloutAnchor, false);
2. calloutAnchor near a vertical edge
3. alter calloutAnchor.x
4. callout.updatePopUpPosition();

Actual Results:
The arrow position is not updated if the callout doesn't move (seems that the display list is not invalidated although the arrow needs to move).

Expected Results:
Arrow position updates whenever host position changes. Also when callout position doesn't change.

Workaround (if any):
Explicitly invalidate the display list whenever updatePopUpPosition() is called:

FixedCallout extends Callout
{
override public function updatePopUpPosition():void

}