Another useful plugin for Joplin - Ridgeline. A whole sidebar for something you glance at a few times an hour always felt like a bad trade, so this one squeezes the outline into a thin stack of bars at the editor’s edge: one bar per heading, longer for higher levels, the current section highlighted. Rest the mouse on it - the full table of contents unfolds. Click a heading - you’re there. Move away - it folds back into a few quiet pixels.
Works in both the Markdown editor and the rendered view, follows your theme, can float over the text or reserve a small margin - whichever you prefer. Notes without headings show nothing at all.
Free and open source: github.com/pmslava/joplin-plugin-ridgeline - installable from Joplin’s plugin search.