The Bootstrap documentation site uses a very interesting navigation concept.
They have a per-page navigation bar that sticks on top of the page once you scroll down and highlights the current area on the page as you go.
Here is a screenshot of the navigation bar when you are at the top of the page.
The red rectangle is not part of the design, it is used to highlight what navigation bar I'm referring to.
This is a screenshot of the same page but scrolled down a bit.
Note how that navigation bar suddenly stretches across the entire width of the page and always sticks at the top.
Is there a name for this navigation concept, if so, what is it called?