Ok, so it seems that to set a default snippet option, you have to let its `data-select_class` empty.

Here's an example:

<div data-selector="section" data-js="snippet_foobar">
    <li class="dropdown-submenu">
        <a href="#">Foobar</a>
        <ul class="dropdown-menu">
            <li data-select_class="opt-foo">
                <a>Foo</a>
            </li>
            <li data-select_class="opt-bar">
                <a>Bar</a>
            </li>
            <li data-select_class="">
                <a>When visible</a>
            </li>
        </ul>
    </li>
</div>