The Canvas class can draw outside its bounds if you issue a draw command, then update its bounds afterwards via a binding to width and height. Example that draws a visible red line on Windows 7 with 8u5:

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import javafx.application.Application;
import javafx.beans.property.SimpleDoubleProperty;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.canvas.Canvas;
import javafx.scene.canvas.GraphicsContext;
import javafx.scene.paint.Color;
import javafx.stage.Stage;

public class TestCanvas extends Application
{

    @Override
    public void start(Stage primaryStage) throws Exception {
        Canvas c = new Canvas(0, 0);
        SimpleDoubleProperty width = new SimpleDoubleProperty(0);
        SimpleDoubleProperty height= new SimpleDoubleProperty(0);
        c.widthProperty().bind(width);
        c.heightProperty().bind(height);
        
        GraphicsContext gc = c.getGraphicsContext2D();
        
        gc.setLineWidth(0.75);
        gc.setStroke(Color.RED);
        gc.strokeLine(0, 10, 40, 10);
        
        
        primaryStage.setScene(new Scene(new Group(c)));
        primaryStage.show();
        width.set(50);
        height.set(40);
    }

}
=====

It draws a red line on a canvas of size 0x0, then resizes the canvas to reveal a red line. I think it must be to do with the binding aspect -- if I use setWidth and setHeight directly at the end of the method instead of the width binding as above, I don't see the same problem.