FULL PRODUCT VERSION :
java version "1.8.0_72"
Java(TM) SE Runtime Environment (build 1.8.0_72-b15)
Java HotSpot(TM) 64-Bit Server VM (build 25.72-b15, mixed mode

ADDITIONAL OS VERSION INFORMATION :
Yosemite 10.10.5


A DESCRIPTION OF THE PROBLEM :
I am displaying a web page in a browser, when the page generates a javascript popup, we catch the event and try to JOptionPane.showConfirmDialog and the whole app locks up. Only occurs on mac.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
I wrote a sample app, run it, click the button will launch a browser page, in the browser page there is a try it button that will generate a event, click that button locks up the whole app


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
the app not to lock up and display my JOptionPane.showConfirmDialog
ACTUAL -
Application Locks up

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
//----------------Test.java----------------------


import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;



public class Test {

    private class MainFrame extends JFrame
    {
        JButton openWindowButton = new JButton("Click Me");

       public MainFrame(){
          super("Test Frame");
           setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
           openWindowButton.setPreferredSize(new Dimension(500,500));
           getContentPane().add(openWindowButton, BorderLayout.CENTER);
           pack();
           setVisible(true);
           openWindowButton.addActionListener(new ActionListener() {
               public void actionPerformed(final ActionEvent ev) {
                Viewer view = new Viewer(MainFrame.this);
                   try{
                       view.loadURL("http://www.w3schools.com/js/tryit.asp?filename=tryjs_alert");}
                   catch (Exception e){
                       System.out.println(e.getMessage());
                   }
                   view.pack();
                   view.setVisible(true);


               }
           });
       }

    }

    private class Viewer extends JDialog{

        private JButton closeButton = new JButton("Close");
        private Browser browser;
        public Viewer(Window owner){
            super(owner, "Viewer", ModalityType.APPLICATION_MODAL);
            JComponent contentView = initUI();

            MStandardDialogPanel decoratedPane = new MStandardDialogPanel(null,
                    MStandardDialogPanel.SeparatorStyle.BOTTOM, MStandardDialogPanel.BodyPaneInsetStyle.NONE);
            decoratedPane.setMinimumButtonWidth(new Double(100));
            decoratedPane.addButton(closeButton,SwingConstants.CENTER);
            decoratedPane.setBodyPane(contentView);
            decoratedPane.setBorder(BorderFactory.createEmptyBorder(10, 15, 10, 15));
            this.add(decoratedPane, BorderLayout.CENTER);


        }
        public void loadURL(String url){

            try{
                browser.loadURL(url);}
            catch (Exception e){
                System.out.println(e.getMessage());
            }

        }
        private JComponent initUI() {

            JPanel contentPane = new JPanel();
            contentPane.setLayout(new GridBagLayout());
            contentPane.setPreferredSize(new Dimension(800,600));
            browser= new Browser();
            JComponent browserPane = browser.getWebBrowserPanel();
            contentPane.add(browserPane, new GridBagConstraints(0, 0, 1, 1, 1, 1, GridBagConstraints.WEST, GridBagConstraints.BOTH, new Insets(0, 0, 0, 0), 0, 0));
            return contentPane;
        }
    }

    public Test(){
        new MainFrame();
    }

    public static void main(String [] args){
        Test thisTest = new Test();

    }
}

//---------------- End Test.java----------------------





//----------------Browser.java-----------------------

import java.awt.*;
import java.awt.event.WindowEvent;
import java.net.MalformedURLException;
import java.net.URL;
import javax.swing.*;
import javafx.application.Platform;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.concurrent.Worker;
import javafx.embed.swing.JFXPanel;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.web.WebEngine;
import javafx.scene.web.WebEvent;
import javafx.scene.web.WebView;
import javafx.util.Callback;



public class Browser {

    private final JFXPanel jFXPanel;
    private WebEngine webEngine;

    public Browser() {

        jFXPanel = new JFXPanel();
        jFXPanel.setVisible(true);
        createScene();
        Platform.setImplicitExit(false);
    }

    private void createScene() {

        Platform.runLater(new Runnable() {
            // The Scene must be created on the JavaFX thread
            @Override
            public void run() {

                final WebView webView = new WebView();
                webView.setContextMenuEnabled(false);
                webEngine = webView.getEngine();

                jFXPanel.setScene(new Scene(webView));
            }
        });
    }
    public void loadErrorPage() {

        final String ERROR_PAGE = "<html><body><h4>Your request cannot be completed, please call support for assistance.</h4></body></html>";

        Platform.runLater(new Runnable() {
            @Override
            public void run() {
                webEngine.loadContent(ERROR_PAGE);
            }
        });
    }

    public void loadURL(final String url) throws MalformedURLException {

        final String urlExternalForm = new URL(url).toExternalForm();

        Platform.runLater(new Runnable() {

            // Forwarded location flag (YM)
            boolean loadedForwardedLocation = false;
            int loadWorkerCount = 0;

            // The URL must be loaded on the JavaFX thread
            @Override
            public void run() {

                webEngine.getLoadWorker().stateProperty().addListener(new ChangeListener<Worker.State>() {
                    @Override
                    public void changed(@SuppressWarnings("rawtypes") ObservableValue ov, Worker.State oldState, Worker.State newState) {
                        if (newState == Worker.State.SUCCEEDED) {

                            // For Java 7: check whether load worker has loaded twice per single url request (YM)
                            if(System.getProperty("java.runtime.version", "").startsWith("1.7")) {
                                if(!loadedForwardedLocation) {
                                    loadWorkerCount++;

                                    // If so, reload current location to set corresponding view state (YM)
                                    if(loadWorkerCount == 2) {
                                        webEngine.load(webEngine.getLocation());
                                        loadedForwardedLocation = true;
                                    }
                                }
                            }
                        }
                        // Load error page if target location request fails (YM)
                        else if (newState == Worker.State.FAILED) {
                            loadErrorPage();
                        }
                    }
                });

                // Capture and display JavaScript alert messages (YM)
                webEngine.setOnAlert(new EventHandler<WebEvent<String>>() {
                    @Override
                    public void handle(final WebEvent<String> event) {

                            JOptionPane.showConfirmDialog(jFXPanel, "Test");
                    }
                });


                if (urlExternalForm != null) {
                    webEngine.load(urlExternalForm);
                }
            }
        });
    }

    public JFXPanel getWebBrowserPanel() {
        return jFXPanel;
    }
    /*----------- Close window that contains this browser instance (YM)-----------*/
    private void closeBrowserContainer() {
        JDialog container = (JDialog) SwingUtilities.getWindowAncestor(jFXPanel);
        container.dispatchEvent(new WindowEvent(container, WindowEvent.WINDOW_CLOSING));
        container.dispose();
    }

}


//------------------End Browser.java------------------------------


//-----------------MStandardDialogPanel.java ------------------


import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;

import javax.swing.AbstractButton;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JComponent;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JSeparator;
import javax.swing.SwingConstants;
import javax.swing.border.Border;

public class MStandardDialogPanel
        extends JPanel {

    public static enum SeparatorStyle {
        TOP,
        BOTTOM,
        BOTH,
        NONE
    }

    public static enum BodyPaneInsetStyle {
        NORMAL,
        WIDE,
        NONE
    }

    public static final int OUTER_PADDING_HORIZONTAL = 16;
    public static final int OUTER_PADDING_VERTICAL = 16;
    public static final Border OUTER_BORDER = BorderFactory.createEmptyBorder(
            OUTER_PADDING_VERTICAL, OUTER_PADDING_HORIZONTAL,
            OUTER_PADDING_VERTICAL, OUTER_PADDING_HORIZONTAL);

    private static final long serialVersionUID = 1L;
    private HeaderPane headerPane;
    private JPanel bodyPane;
    private FooterPane footerPane;

    public MStandardDialogPanel() {
        this(null, SeparatorStyle.NONE, BodyPaneInsetStyle.NORMAL);
    }

    public MStandardDialogPanel(final String titleText) {
        this(titleText, SeparatorStyle.TOP, BodyPaneInsetStyle.NORMAL);
    }

    public MStandardDialogPanel(final SeparatorStyle separatorStyle) {
        this(null, separatorStyle, BodyPaneInsetStyle.NORMAL);
    }

    public MStandardDialogPanel(final SeparatorStyle separatorStyle, final BodyPaneInsetStyle bodyPaneInsetStyle) {
        this(null, separatorStyle, bodyPaneInsetStyle);
    }

    public MStandardDialogPanel(final String titleText, final SeparatorStyle separatorStyle) {
        this(titleText, separatorStyle, BodyPaneInsetStyle.NORMAL);
    }

    public MStandardDialogPanel(final String titleText,
                                final SeparatorStyle separatorStyle,
                                final BodyPaneInsetStyle bodyPaneInsetStyle) {
        {
            setLayout(new BorderLayout(0, 0));
            setBorder(OUTER_BORDER);
            setBackground(Color.white);

            // do we need to add a header pane?
            if (titleText != null || separatorStyle == SeparatorStyle.TOP ||
                    separatorStyle == SeparatorStyle.BOTH) {
                headerPane = new HeaderPane(titleText, separatorStyle);
                add(headerPane, BorderLayout.NORTH);
            }

            // always add a body and a footer pane (for the buttons)
            bodyPane = new JPanel(new BorderLayout(0, 0));
            bodyPane.setOpaque(false);
            if (bodyPaneInsetStyle == BodyPaneInsetStyle.NORMAL) {
                bodyPane.setBorder(BorderFactory.createEmptyBorder(10, 15, 10, 15));
            } else if (bodyPaneInsetStyle == BodyPaneInsetStyle.WIDE) {
                bodyPane.setBorder(BorderFactory.createEmptyBorder(10, 0, 10, 0));
            }

            footerPane = new FooterPane(separatorStyle);

            add(bodyPane, BorderLayout.CENTER);
            add(footerPane, BorderLayout.SOUTH);
        }
    }

    public MStandardDialogPanel setTitle(String titleText) {
        if (headerPane == null) {
            headerPane = new HeaderPane(titleText, SeparatorStyle.NONE);
            add(headerPane, BorderLayout.NORTH);
        } else {
            headerPane.setTitle(titleText);
        }
        return this;
    }

    public MStandardDialogPanel setBodyPane(Component bodyComponent) {
        bodyPane.add(bodyComponent);
        return this;
    }

    public MStandardDialogPanel setBodyPane(Component bodyComponent, int index) {
        bodyPane.add(bodyComponent, index);
        return this;
    }

    public MStandardDialogPanel addButton(AbstractButton button, int alignment) {
        footerPane.add(button, alignment);
        return this;
    }

    public MStandardDialogPanel addButton(AbstractButton button) {
        footerPane.add(button, SwingConstants.RIGHT);
        return this;
    }

    public MStandardDialogPanel addComponent(JComponent component, int alignment) {
        footerPane.add(component, alignment);
        return this;
    }

    public MStandardDialogPanel setMinimumButtonWidth (Double width) {
        footerPane.setMinimumButtonWidth(width);
        return this;
    }

    @Override
    public Component add(Component comp) {
        setBodyPane(comp);
        return comp;
    }

    @Override
    public Component add(Component comp, int index) {
        setBodyPane(comp, index);
        return comp;
    }


    private class HeaderPane extends JPanel {
        /**
         *
         */
        private static final long serialVersionUID = 1L;

        private JLabel titleLabel;

        private HeaderPane(final String title, final SeparatorStyle separatorStyle) {

            setOpaque(false);

            GridBagLayout gridBagLayout = new GridBagLayout();
            gridBagLayout.columnWidths = new int[] { 1, 0 };
            gridBagLayout.rowHeights = new int[] { 8, 0, 0 };
            gridBagLayout.columnWeights = new double[] { 1.0, Double.MIN_VALUE };
            gridBagLayout.rowWeights = new double[] { 0.0, 0.0, Double.MIN_VALUE };
            setLayout(gridBagLayout);


            titleLabel = new JLabel(title);
            titleLabel.setForeground(Color.BLACK); // CHANGE THIS
            titleLabel.setFont(getFont().deriveFont(Font.PLAIN, getFont().getSize() * 1.5f));
            titleLabel.setHorizontalAlignment(SwingConstants.LEFT);

            GridBagConstraints gbc_titleLbl = new GridBagConstraints();
            //gbc_titleLbl.ipady = 4;
            //gbc_titleLbl.ipadx = 16;
            gbc_titleLbl.weightx = 1.0;
            gbc_titleLbl.fill = GridBagConstraints.BOTH;
            gbc_titleLbl.insets = new Insets(0, 0, 7, 0);
            gbc_titleLbl.anchor = GridBagConstraints.WEST;
            gbc_titleLbl.gridx = 0;
            gbc_titleLbl.gridy = 0;
            add(titleLabel, gbc_titleLbl);

            if (separatorStyle == SeparatorStyle.TOP || separatorStyle == SeparatorStyle.BOTH) {
                JSeparator separator = new JSeparator();
                separator.setForeground(Color.ORANGE);
                GridBagConstraints gbc_separator = new GridBagConstraints();
                gbc_separator.fill = GridBagConstraints.HORIZONTAL;
                gbc_separator.insets = new Insets(0, 0, 7, 0);
                gbc_separator.gridwidth = 2;
                gbc_separator.weightx = 1.0;
                gbc_separator.anchor = GridBagConstraints.NORTHWEST;
                gbc_separator.gridx = 0;
                gbc_separator.gridy = 1;
                add(separator, gbc_separator);
            }

        }

        public void setTitle(final String titleText) {
            titleLabel.setText(titleText);
        }

    }


    private class FooterPane extends JPanel {
        /**
         *
         */
        private static final long serialVersionUID = 1L;

        final JPanel left;
        final JPanel center;
        final JPanel right;
        private Double minimumButtonWidth;

        private FooterPane(final SeparatorStyle separatorStyle) {

            /*
setLayout(new BorderLayout(0, 0));

            if (separatorStyle == SeparatorStyle.BOTTOM || separatorStyle == SeparatorStyle.BOTH) {
JSeparator separator = new JSeparator();
separator.setForeground(Color.ORANGE);
add(separator, BorderLayout.NORTH);
            }

add(left, BorderLayout.WEST);
add(center, BorderLayout.CENTER);
add(right, BorderLayout.EAST);
left.setOpaque(false);
right.setOpaque(false);
center.setOpaque(false);
            */

            setOpaque(false);

            GridBagLayout gridBagLayout = new GridBagLayout();
            gridBagLayout.columnWidths = new int[] { 1, 0 };
            gridBagLayout.rowHeights = new int[] { 0, 1, 0 };
            gridBagLayout.columnWeights = new double[] { 1.0, Double.MIN_VALUE };
            gridBagLayout.rowWeights = new double[] { 0.0, 1.0, Double.MIN_VALUE };
            setLayout(gridBagLayout);


            if (separatorStyle == SeparatorStyle.BOTTOM || separatorStyle == SeparatorStyle.BOTH) {
                JSeparator separator = new JSeparator();
                separator.setForeground(Color.ORANGE);
                GridBagConstraints gbc_separator = new GridBagConstraints();
                gbc_separator.fill = GridBagConstraints.HORIZONTAL;
                gbc_separator.insets = new Insets(7, 0, 0, 0);
                gbc_separator.gridwidth = 2;
                gbc_separator.weightx = 1.0;
                gbc_separator.anchor = GridBagConstraints.SOUTHWEST;
                gbc_separator.gridx = 0;
                gbc_separator.gridy = 0;
                add(separator, gbc_separator);
            }


            final JPanel buttonPanel = new JPanel(new BorderLayout(0, 0));
            left = new JPanel(new FlowLayout(FlowLayout.LEFT));
            center = new JPanel(new FlowLayout(FlowLayout.CENTER));
            right = new JPanel(new FlowLayout(FlowLayout.RIGHT));

            buttonPanel.setOpaque(false);
            left.setOpaque(false);
            right.setOpaque(false);
            center.setOpaque(false);

            buttonPanel.add(left, BorderLayout.WEST);
            buttonPanel.add(center, BorderLayout.CENTER);
            buttonPanel.add(right, BorderLayout.EAST);


            GridBagConstraints gbc_buttonPanel = new GridBagConstraints();
            //gbc_buttonPanel.ipady = 4;
            //gbc_buttonPanel.ipadx = 16;
            gbc_buttonPanel.weightx = 1.0;
            gbc_buttonPanel.fill = GridBagConstraints.BOTH;
            gbc_buttonPanel.insets = new Insets(7, 0, 0, 0);
            gbc_buttonPanel.anchor = GridBagConstraints.SOUTH;
            gbc_buttonPanel.gridx = 0;
            gbc_buttonPanel.gridy = 1;
            add(buttonPanel, gbc_buttonPanel);

        }

        public void add(AbstractButton button, final int alignment) {

            // see if we need to apply a minimum width to the button
            if (this.minimumButtonWidth != null) {

                // if it's any other kind of button, we'll resize it here (but changing the text later
                // might cause the button to be resized to a smaller size)
                Dimension buttonSize = button.getPreferredSize();
                if (buttonSize.getWidth() < this.minimumButtonWidth) {
                    buttonSize.setSize((double) this.minimumButtonWidth, buttonSize.getHeight());
                    button.setPreferredSize(buttonSize);
                }
            }


            switch (alignment) {
                case SwingConstants.LEFT:
                    left.add(button);
                    break;
                case SwingConstants.CENTER:
                    center.add(button);
                    break;
                case SwingConstants.RIGHT:
                    right.add(button);
                    break;
                default:
                    break;
            }
        }

        public void add(JComponent component, final int alignment) {

            switch (alignment) {
                case SwingConstants.LEFT:
                    left.add(component);
                    break;
                case SwingConstants.CENTER:
                    center.add(component);
                    break;
                case SwingConstants.RIGHT:
                    right.add(component);
                    break;
                default:
                    break;
            }
        }

        public void setMinimumButtonWidth (Double width) {
            this.minimumButtonWidth = width;
        }

    }

   
    /**
     * Method will place find button on header of dialog.
     *
     * @param findButton
     */
    public void setFindButtonOnHeader(JButton findButton) {
        if(headerPane!=null){
            headerPane.add(findButton);
        }
    }
}


//-----------------End MStandardDialogPanel.java---------------------

---------- END SOURCE ----------