Why do my group nodes jump to the scene root's origin?

Question

The code for my project is too large to post, so I'll try to explain the problem I'm experiencing. I'm creating a graph layout tool as part of larger visualization tool we are building.
The problem I'm having is a group I have positioned using layoutX, layoutY is jumping to the origin whenever we do the graph layout.
Here is the basic flow of the program:
We create a JUNG graph of Circle(vertex) and Lines(edge)
We use JUNG's layout classes to generate a layout for the graph. The layout generates an x, y position for each vertex in the graph within a defined area. Say a 400 by 400 box.
We use the generated positions to set the centerX, centerY of the circles. The lines are bound to the centres of the nodes they go between, so they update automatically.
This is all done to present the initial graph layout. The user is able to select nodes and choose them to be laid out in a new way. This creates a sublayout on the canvas. When the circles are selected we move them to a group position in the exact spot where they were selected. The groups are made draggable and can be moved around the canvas. The lines update, and the circles do not move relative to their parent group's coordinate system. Only their container is moved around inside sceneRoot.
The scengraph looks like:
VBox
Group:sceneRoot
Circles and Lines making up the original graph
Group: sublayout
The selected Circles and Lines
The problem comes when we try to do the layout of the selected nodes. We generate a JUNG subgraph. Then we do the graph layout with the subgraph to give the circles it contains new positions. We ask the layout to generate a layout the size of sublayout group. This works. JUNG generates new values for the positions of the Circles. When we set the centers of the circles, the group jumps to the upper left hand corner - the sceneRoot's origin. In my understanding, the group shouldn't move at all. It's layoutX,layoutY have been set and the circles are being positioned relative to sublayout's origin. I can't figure out why the group is jumping to the origin. It should be staying in place.

Answer 1

I figured it out!
The problem was using a group as the root of the canvas. A Group's bounds are the sum of its children's bounds. This means that a group's bounds will change depending on what is in it. When the VBox laid it out, it would move because of the new bounding box. The fix was to use a Pane. A Pane resizes according to the pref/min/max rules and does not shrink to be the sum of its children. It is a more "solid" container.