[Accordion] SceneBuilder side panel not fully visible

To reproduce:

1) Run SceneBuilderApp

Note that you can only see the "Containers" tab in the left side panel. You cannot see the Controls, Menu, etc., tabs. Additionally, if you scroll down the list of containers you cannot get to the bottom (which should be VBox). See the "before" / after picture for what it should look like versus what it looks like today.

This is a serious regression introduced in the past week in either Accordion or layout in general. 8u20-b14 works fine. Today's build fails. Note that whether I use last week's SceneBuilder or this week's SceneBuilder doesn't matter. The failure is caused by some change in the FX runtime.