See ignored test in ExtendedWadlWebappTest.testWadlOptionsMethod().
When MBW is invoked to serialize com.sun.research.ws.wadl.Application to the response stream it fails during marshalling of org.glassfish.jersey.server.wadl.internal.generators.resourcedoc.xhtml.XhtmlElementType. JAXBContext does not have classes from the package org.glassfish.jersey.server.wadl.internal.generators.resourcedoc.xhtml. These classes needs to be passed to JAXBContext from WadlGeneratorResourceDocSupport.
The output xml is partially written (known classes are serialized) but fails in the middle of serialization. When there is no buffering of the response the test passes as 500 cannot be returned after first bytes are written to the output. The test is bad as it does not properly check returned xml on the client side. When buffering is present 500 is returned and the test fail.
This problem is there for longer time but thanks to missing buffering and bad test it was just ignored.
This issue is not about the buffering but to fix the extended wadl serialization.
ExtendedWadlWebappOsgiTest.testWadlOptionsMethod fails probably due to same problem (ignored now).