import com.gargoylesoftware.htmlunit.WebClient;
...
import java.util.LinkedList;

public class GeekMailSender {
    ...
    
    // Static variable to track all website windows.
    private final static LinkedList<WebWindow> websiteWindows = new LinkedList<WebWindow>();
    
    // Inner-class to listen for new (i.e. pop-up) windows.
    static class GeekMailWindowListener implements WebWindowListener {
        public void webWindowClosed(WebWindowEvent event) {}
        public void webWindowContentChanged(WebWindowEvent event) {}
        public void webWindowOpened(WebWindowEvent event) {
            GeekMailSender.websiteWindows.add(event.getWebWindow());
        }
    }

    // Method to actually send GeekMail by navigating the BGG website.
    public static void sendGeekMail(...) {
        ...
        try (final WebClient webClient = new WebClient()) {
            // Track creation of new (i.e. pop-up) windows.
            websiteWindows.clear();
            webClient.addWebWindowListener(new GeekMailWindowListener());
            	
            // Try to access the GeekMail page.
            HtmlPage currentPage = webClient.getPage("https://boardgamegeek.com/geekmail/compose?touser=FakeUserName");
            String pageTitle = currentPage.getTitleText();
            System.out.println(pageTitle);  // BoardGameGeek
            
            // We may need to login first.
            if (!pageTitle.contains("GeekMail")) {
            	// Need to wait for javascript to complete, otherwise no forms are available.
            	webClient.waitForBackgroundJavaScriptStartingBefore(JAVASCRIPT_PAUSE);
            	// No difference if use webClient.waitForBackgroundJavaScript(JAVASCRIPT_PAUSE);

                // Unfortunately the only form found is the top-right Search form on BoardGameGeek.
                if (currentPage.getForms().isEmpty()) {
                        // This does NOT happen.
            		System.out.println("WARNING! No form found, even after waiting for javascript!");
            		return;
            	}

                // We don't find any windows at all... this confuses me.
            	if (websiteWindows.isEmpty()) {
                        // This does happen :(
            		System.out.println("WARNING! No windows found even after waiting for javascript!");
            	}

                // Additional printing does not reveal where the form is.
                // For instance, searching the XML for "Sign up" yields no results.
            	System.out.println(currentPage.asXml());
            	
            	// And printing the one form we can access reveals it is just the Search form.
            	System.out.println(currentPage.getForms().size());  // 1
            	final HtmlForm loginForm = currentPage.getForms().get(0);
            	System.out.println(loginForm.asXml());
                ...
            }
            ...
        }
        ...
    }