Modern apps like reader (beta) by readwise use the left pane to display the outline H1, H2, H3 (see screenshot).
Why not to implement this in Coda. You could toggle on the outline (see red arrow). Moving back (green arrow), the outline disapears.
We already have /outline. It will give you the headings, as well as tables on the right hand side of the doc.
I know this. And it is a pane. It reduces the space in the main window. I propose a better implementation of the outline and get this reply.