Group widgets are not customizable

Expected behavior

When I use group-by="type" directive on a solid-display component, object are grouped by type. The generated markup is

<div>
  <span data-title="">http://localhost:8000/typeevents/1/</span>
  <div data-content="">
      <solid-display>
        ...
      </solid-display>
  </div>
</div>

I would like to customize it, so I try to add a group-by-widget="h2" directive, but that does not work.

What's happening?

Nothing, the group-by-widget directive seams to have no effect.

Your Environment

(Include relevant details about the environment you experienced the bug in)

• Library version: sib core@0.12.3

• Browser name and version: Firefox 81.0

• Operating System and version (desktop or mobile): Ubuntu

• Targeted application: Hubl