The infolines theme shows the section and subsection titles in the header, in one single line, preceding the title for the frame (in the second line).
I'd like to show also the titles of the previous and next section and subsection (in a transparent way) to give some context, similarly to what the split header theme does (but restricted to only the previous and next elements).
I think I have seen this somewhere but only on the PDF not in the code.
How can this be achieved?