The WF testsuite includes a test-feature-pack which provide a Galleon feature pack with layers intended to mirror higher level layers that will be used in the WildFly s2i image, with the goal to make those easy to use in WildFly testsuite tests of slimmed servers. See
WFLY-12507 and WFLY-12304.
One of these layers is 'web-clustering' which is meant to decorate a server provisioned with an undertow web server to add web clustering functionality.
Testing reveals a server provisioned with this combination is missing the org.wildfly.cluster.web.undertow module. This is because the chain of module deps from the distributable-web extension module does not force its inclusion, because 'undertow' is an optional impl detail. And the chain of module deps from the undertow extension module does not force its inclusion, because clustering is an optional feature.
So something needs to force its inclusion, and a way I see to do it is to have the web-clustering layer do it. That layer knows it wants undertow and it knows it wants clustering.