Extending Container for custom layout not working properly

Question

I'm trying to extend javafx.scene.layout.Container to make a custom layout manager that centers a grid of components and fills all available area in the stage (minus some insets, eventually). I'm using some private members to do this, but it's also being done by the JFXtras library, for example. (Incidentally, I would use the new MigLayout in JFXtras 0.3, but it may have an issue that doesn't allow me to do what I'm trying to do, see [http://groups.google.com/group/jfxtras-users/browse_thread/thread/a26bb47bbbc710a0]). Here's what I'm expecting from the layout:
| 1 2 |
| 3 4 |
and after a resize something like:
|           |
|   1   2   |
|           |
|   3   4   |
|           |
-------------So far, my Container implementation is getting there, but I need to resize the window once after the application starts up to get it to work right. Right after it starts, the nodes are all drawn at 0,0. It appears as though scene.width and scene.height are only initialized properly after my impl_layout function is called the first time, but I'm relying on these to calculate the width and height of the scene/stage to do the centering. If I instead rely on scene.stage.width and scene.stage.height, it works right at startup but doesn't work properly when resizing.
What should I do to fix my layout method? Is there a better way to handle this layout? HBox and VBox don't give me what I want because they don't set the positions of the components to fill the available screen area. I'm using JavaFX 1.1 on OS X.
----- GridContainer.fx -----
import javafx.scene.Group;
import javafx.scene.layout.Container;
public class GridContainer extends Container {
    public var rows: Integer = 3;
    public var columns: Integer = 3;
    init {
        impl_layout = doGridLayout;
    function doGridLayout(g:Group):Void {
        println("width = {g.scene.width}");
        println("height = {g.scene.height}");
        def tXIncrement = g.scene.width / (columns + 1);
        def tYIncrement = g.scene.height / (rows + 1);
        var tX: Number = tXIncrement;
        var tY: Number = tYIncrement;
        var r: Integer = 0;
        var c: Integer = 0;
        for (node in g.content) {
            node.translateX = tX - (node.boundsInLocal.width / 2);
            node.translateY = tY - (node.boundsInLocal.height / 2);
            if (++c == columns) {
                tX = tXIncrement;
                c = 0;
                if (++r == columns)
                    break;
                tY += tYIncrement;
            else {
                tX += tXIncrement;
----- Main.fx -----
import javafx.scene.paint.Color;
import javafx.scene.Scene;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;
def width = 640;
def height = 480;
Stage {
    title: "Layout Test"
    width: width
    height: height
    scene: Scene {
        content: [
            GridContainer {
                rows: 2,
                columns: 2,
                content: [
                    Rectangle {
                        width: 150,
                        height: 100,
                        fill: Color.BLACK
                    Rectangle {
                        width: 150,
                        height: 100,
                        fill: Color.RED
                    Rectangle {
                        width: 150,
                        height: 100,
                        fill: Color.GREEN
                    Rectangle {
                        width: 150,
                        height: 100,
                        fill: Color.BLUE
    } // Scene
} // Stage
----------

Answer 1

Incidentally, MigLayout is capable of doing this properly, but I had to create MigNode objects for each cell (see discussion link above).
I'm still interested in how to do this with a custom Container, because I may end up using my own layout code in the future anyway...