A DESCRIPTION OF THE PROBLEM :
When a new item with (ResizableWithParent = false) is added and divider positions are set, SplitPane ignores specified positions.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the provided code and press the button.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The added pane must be visible and the output of the program should be:
Current pulse: [0.5]
2nd pulse: [0.5]
3rd pulse: [0.5]

ACTUAL -
The added pane is not visible and the output of the program is:
Current pulse: [0.5]
2nd pulse: [0.5]
3rd pulse: [0.9797297297297297]


---------- BEGIN SOURCE ----------
public class TestFx extends Application {

    public static void main(String[] args) {
        launch(args);
    }

    private final SplitPane splitPane = new SplitPane();

    @Override
    public void start(Stage primaryStage) {
        splitPane.getItems().add(createItem(0));
        splitPane.setOrientation(Orientation.VERTICAL);
        VBox.setVgrow(splitPane, Priority.ALWAYS);
        var fooPane = new Pane(new Label("Foo pane"));
        VBox.setVgrow(fooPane, Priority.ALWAYS);

        var button = new Button("Test");
        button.setOnAction(e -> {
            splitPane.getItems().add(createItem(1));
            splitPane.setDividerPositions(new double[]{0.5});
            printDividerPositions("Current pulse: ");
            Platform.runLater(() -> printDividerPositions("2nd pulse: "));
            Platform.runLater(() -> Platform.runLater(() -> printDividerPositions("3rd pulse: ")));
        });

        var root = new VBox(splitPane, fooPane, button);
        Scene scene = new Scene(root, 500, 300);
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    private void printDividerPositions(String pulseInfo) {
        System.out.println(pulseInfo + Arrays.toString(splitPane.getDividerPositions()));
    }

    private Pane createItem(int index) {
        var pane = new Pane(new Label("Item : " + index));
        if (index == 0) {
            SplitPane.setResizableWithParent(pane, true);
        } else {
            SplitPane.setResizableWithParent(pane, false);
        }
        return pane;
    }
}
---------- END SOURCE ----------