Why does Html.ActionLink render "?Length=4"

The Length=4 is coming from an attempt to serialize a string object. Your code is running this ActionLink method:

public static string ActionLink(this HtmlHelper htmlHelper, string linkText, string actionName, object routeValues, object htmlAttributes)

This takes a string object "Home" for routeValues, which the MVC plumbing searches for public properties turning them into route values. In the case of a string object, the only public property is Length, and since there will be no routes defined with a Length parameter it appends the property name and value as a query string parameter. You'll probably find if you run this from a page not on HomeController it will throw an error about a missing About action method. Try using the following:

Html.ActionLink("About", "About", new { controller = "Home" }, new { hidefocus = "hidefocus" })

The way I solved this is was adding a null to the fourth parameter before the anonymous declaration (new {}) so that it uses the following method overload: (linkText, actionName, controllerName, routeValues, htmlAttributes):

Html.ActionLink("About", "About", "Home", null, new { hidefocus = "hidefocus" })