I believe you'd describe it as a pattern. What is the name of the pattern when you scroll down and the action bar hides to show more space and reveals when you scroll back up. Thanks.
This sounds like an example of progressive disclosure in interaction design for mobile.
I doubt if there's a specific name for this pattern. As far as iOS is concerned, it was achieved using the viewport meta tag known as minimal-ui. The minimal-ui viewport was supported till iOS 7.1 and was removed in iOS 8. Although, the minimal-ui is not gone. It still works when you scroll down.
I see it called an "Animated Sticky Header/Footer" or an "Animated Resizing Header/Footer." A "sticky header/footer" by itself remains in place regardless of scrolling. If it changes shape when scrolled past a certain point, it's "animated" or "resizing."
The top-most header elements in a site (the title, logo, nav, etc) are initially displayed within a vertically taller section at the top of the site. Rather than scroll entirely out of view, they remain visible at the top of the page, shrinking to a smaller vertical height, with links & logos shrinking as well or perhaps being hidden behind a "hamburger" menu. This is common behavior for many Wordpress themes & responsive sites that want to keep nav/header elements accessible on mobile devices.
I've implemented these a few times. I haven't come across a standard name for them.
If you are doing a search (eg looking for plug-ins or implementation tips) you can use the term
scroll up menuin Google and you'll be well on your way.
If you are looking for a naming convention (e.g. for CSS or JS work) I found the name
scroll up menuquite long and unwieldy in practice so I've used the name
upscrollerin code before.