Mixing a griffon view + swing toolkit + javafx. It works, but you're not using a builder for the javafx nodes.

griffon+swing+javafx+platform

package testmixfx
 
import javafx.embed.swing.JFXPanel
import groovyx.javafx.GroovyFX
import javafx.application.Platform;
import javafx.embed.swing.JFXPanel;
import javafx.scene.Scene;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebView;
import groovyx.javafx.SceneGraphBuilder;
 
 
JFXPanel jfxPanel = new JFXPanel();
def DEFAULT_URL = "http://www.yahoo.com"
 
 
application(title: 'testmixfx',
  preferredSize: [320, 240],
  pack: true,
  locationByPlatform: true,
  iconImage:   imageIcon('/griffon-icon-48x48.png').image,
  iconImages: [imageIcon('/griffon-icon-48x48.png').image,
               imageIcon('/griffon-icon-32x32.png').image,
               imageIcon('/griffon-icon-16x16.png').image]) {
    // add content here    
    panel(id:"myPanel",preferredSize: new Dimension(800,400)){
      migLayout(layoutConstraints: 'fill')
      label('Content Goes Here',constraints:"wrap")
      widget(jfxPanel)
    }    
 
    Platform.runLater(new Runnable() {
        @Override
        public void run() {
            final WebView browser = new WebView();
            final WebEngine webEngine = browser.getEngine();
            webEngine.load(DEFAULT_URL);
            Scene s = new Scene(browser);
            jfxPanel.setScene(s);
            webEngine.reload();
        }
    });//--> it works, but it's quite UGLY, no dsl no fun.
 
}