Right now, it’s only possible to collapse lists / headings in Edit mode. This seems like something that should be possible also in the Preview mode.
Also, this ties in to the other request here about “remembering collapsed status.” If it gets implemented in Preview mode, there might be a neat way to remember collapsed status between Preview/Edit modes. Currently, if you collapse a heading/list in Edit, switch to Preview, then switch back, you lose the collapsed view despite not having switched to a different file.
(Possibly a bug report, given expected / current behavior? I’ll leave it here for now.)