Use case or problem
As I write I will add [[#headername]] to remind me that I need to create a header with that name. As the files get larger though I can lose track of what headers are dangling.
Proposed solution
Either add a section to the Outline pane that lists the dangling or missing headers. Or add a 4th pane that, like the outline pane, shows just headers that are referenced in the currently open note but don’t exist.