Oh! I've watched for few seconds more and there is no the first line in the code. That was a bit confusing :')
<a class="navbar-brand" href="/">Navbar</a>
<%= link_to "Navbar", root_path, class: "navbar-brand" %>
with those two lines of code there should be 2 active "Navbar" buttons, those lines are interchangeabale. Somehow after you wrote them you have only 1 button active and I have 2. Is it me or you cut one of those?