Wicket markup inheritance not working for wicket:child tags within MarkupContainers

I'm using markup inheritance in Wicket 6.14.0 to create a simple widget with title and content. The parent's markup looks like this:

<html xmlns="http://www.w3.org/1999/xhtml" xmlns:wicket>
<wicket:panel>
<div wicket:id="widgetBox">
  <div wicket:id="widgetTitle"><wicket:enclosure child="icon"><span class="icon"><i wicket:id="icon"></i></span></wicket:enclosure>
    <h5 wicket:id="headline"></h5>
  </div>
  <div wicket:id="widgetContent"><wicket:child/></div>
</div>
</wicket:panel>
</html>

Its code is pretty straight-forward, but for the sake of completeness:

public class ParentPanel extends Panel {

        public ParentPanel(final String id, final IModel<String> title) {
            super(id, title);       

            WebMarkupContainer widgetBox = new WebMarkupContainer("widgetBox");         
            add(widgetBox);

            final WebMarkupContainer widgetTitle = new WebMarkupContainer("widgetTitle");           
            widgetBox.add(widgetTitle);

            final WebMarkupContainer icon = new WebMarkupContainer("icon");
            if (titleIcon != null) {
                // custom class
                icon.add(new IconBehavior("titleIcon"));
            } else {
                icon.setVisible(false);
            }
            widgetTitle.add(icon);
            widgetTitle.add(new Label("headline", title));

            widgetContent = new WebMarkupContainer("widgetContent");
            widgetContent.add(new WidgetContentBehavior());
            widgetBox.add(widgetContent);
        }
}

Now for the child component. Here's its most simple markup:

<html xmlns="http://www.w3.org/1999/xhtml" xmlns:wicket>
<wicket:extend>
  <span wicket:id="foobar"></span>
</wicket:extend>
</html>

And of course the Java code:

public class ChildPanel extends ParentPanel {

    public ChildPanel(final String id, final IModel<String> title) {
        super(id, title);
        add(new Label("foobar", "Foobar"));
    }

}

What I'm expecting of course is a rendered page saying "Foobar" where the wicket:child tag is placed. Instead there's this RuntimeException:

Last cause: Unable to find component with id 'foobar' in [TransparentWebMarkupContainer [Component id = wicket_extend3]]
    Expected: 'foobar:widgetBox:widgetContent:wicket_child2:wicket_extend3:foobar'.
    Found with similar names: 'foobar', foobar:foobar'

It seems as if markup inheritance does not support wicket:child tags within a MarkupContainer and therefore messes up the hierarchy. I also considered using a Border instead, getting the same outcome though.

My question of course is how to get this work by maintaining the current hierarchy, as it's important to have control (read: a Java Object) of widgetBox and widgetContent.

Solution

Here's another question regarding your issue: Can a wicket:child tag be nested under another Component on the Page?

The answers provided use TransparentWebMarkupContainer to solve the issue.

However, as I found out, note that using TransparentWebMarkupContainer will fail if your child page defines wicket:fragments. See issues WICKET-4545 and WICKET-5440