Once we have composited elements (for example, Button element composited by a Rect, VectorIcon and InvisibleHitTarget elements), it is necessary to have a notion of event bubble/capture.

For instance, in the Button case, in order to position reticle correctly, we need to have an InvisibleHitTarget that is a child of VectorIcon(Button foreground) so that the reticle drew after VectorIcon. This also means all the events are sent to the InvisibleHitTarget instead of Button element (which might be a no hit testable element). But the logic which manages the button state (hover, press) must exist in Button element. So these events need to bubble up to Button element.