I'm trying to use a custom CSS property of type Insets. I have set up a SimpleStyleableObjectProperty<Insets> to hold it, and use StyleConverter.getInsetsConverter() to read it from the CSS file. However, when I set a ChangeListener<Insets> on the property, I get a ClassCastException because the JavaFX runtime is trying to invoke my ChangeListener<Insets>.changed method with a Double as the new value. I think I'm not introducing the Double anywhere myself, but see the code below. Code to reproduce, confirmed exception on Mac OS X 8u5 and 8u20:

import java.util.ArrayList;
import java.util.List;
import java.util.function.Function;

import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.css.CssMetaData;
import javafx.css.SimpleStyleableObjectProperty;
import javafx.css.StyleConverter;
import javafx.css.Styleable;
import javafx.css.StyleableObjectProperty;
import javafx.css.StyleableProperty;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Node;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;


public class TestInsets extends Application
{
    // Sets the VBox insets property via CSS
    private static class BorderPaneCustomInsets extends BorderPane
    {
        private final SimpleStyleableObjectProperty<Insets> frameInsetsProperty = new SimpleStyleableObjectProperty<Insets>(FRAME_INSETS_META_DATA, new Insets(0.0));
        
        public BorderPaneCustomInsets(Node item)
        {
            // If you change <Insets> to <Object> here, you avoid the exception but can see we are being passed a Double:
            frameInsetsProperty.addListener((ChangeListener<Insets>)(a, b, newVal) -> System.out.println(newVal.getClass()));
            getChildren().add(item);
            BorderPane.setAlignment(item, Pos.CENTER);
            getStyleClass().add("myclass");
        }
        
        private static final CssMetaData<BorderPaneCustomInsets, Insets> FRAME_INSETS_META_DATA =
                cssInsets("-my-frame-insets", b -> b.frameInsetsProperty);
             
        private static final List <CssMetaData <? extends Styleable, ? > > cssMetaDataList = new ArrayList<CssMetaData <? extends Styleable, ? >>() {
            {
                addAll(BorderPane.getClassCssMetaData());
                add(FRAME_INSETS_META_DATA);
            }
        };
             
        public static List <CssMetaData <? extends Styleable, ? > > getClassCssMetaData() { return cssMetaDataList; }
        @Override public List<CssMetaData<? extends Styleable, ?>> getCssMetaData() { return getClassCssMetaData(); }
    }
    
    public static <T extends Styleable> CssMetaData<T, Insets> cssInsets(String propertyName, Function<T, StyleableObjectProperty<Insets>> propGetter)
    {
        return new CssMetaData <T, Insets >(propertyName, StyleConverter.getInsetsConverter(), new Insets(0.0)) {
            @Override
            public boolean isSettable(T node)
            {
                return true;
            }

            @Override
            public StyleableProperty <Insets> getStyleableProperty(T node) {
                return propGetter.apply(node);
            }
        };
    }

    @Override
    public void start(Stage stage) throws Exception
    {
        VBox vbox = new VBox();
        
        vbox.getChildren().addAll(new BorderPaneCustomInsets(new Label("Label in pane")) );
        
        Scene scene = new Scene(vbox);
        scene.getStylesheets().add("file:custom-insets.css"); // CSS file which contains:
        // .myclass {
        // -my-frame-insets: 5 0 5 0;
        // }
        stage.setScene(scene);
        
        stage.show();
        
    }
    
}

Thankfully, the workaround is quite simple: use 4 Double properties instead of one Insets until this works.