The snippet for navigation in U8 is generated as followed

@inherits Umbraco.Web.Mvc.UmbracoViewPage
@using Umbraco.Web


@*
    This snippet displays a list of links of the pages immediately under the top-most page in the content tree.
    This is the home page for a standard website.
    It also highlights the current active page/section in the navigation with the CSS class "current".
*@

@{ var selection = Model.Root().Children.Where(x => x.IsVisible()).ToArray(); }

@if (selection.Length > 0)
{
    <ul>
        @foreach (var item in selection)
        {
            <li class="@(item.IsAncestorOrSelf(Model.Content) ? "current" : null)">
                <a href="@item.Url">@item.Name</a>
            </li>
        }
    </ul>
}

Here seems to be a bug. In Umbraco 8 the RenderModel has been removed, so it is no longer Model.Content just Model.

        <li class="@(item.IsAncestorOrSelf(Model.Content) ? "current" : null)">

Therefore correct should be

        <li class="@(item.IsAncestorOrSelf(Model) ? "current" : null)">

Am I right?