Reference Panel: Promote preview pane to main blob view

Created by: scalabilitysolved

Problem

With the new reference panel, it's not always been immediately obvious to me on how to promote the preview panel to render in the main blob view. Often after navigating for a while I'll instead want to see the file in full. On Google Code Search they have a small up arrow icon next to the file link in their preview pane and it feels significant how much more obvious it makes it to a user. Could we explore adding a visual indicator here in a similar fashion?

Another confusing element is how we have the collapse preview pane icon next to the file link which (at least for me) drew my eyes/intuition away from the fact that clicking the file link would actually promote the preview into the main blob view.

Thoughts @jjinnii @macraig ?