remove-sidenav conflicting with horizontal layout

[alandefreitas]

When we remove the sidenav in the antora configuration:

asciidoc:
  attributes:
    # Enable pagination
    page-pagination: ''
    # Remove the sidenav and include TOC in index.adoc page
    remove-sidenav: ''

which is what the playbook for libraries does, the layout does not use the horizontal space.

Here's the output with sitedocs:

And here's the output for libraries:

This is a serious issue as we intend to enable the Antora documentation in the boost release tools soon.