Axismundi Table of Contents provides the axismundi/toc block: a dynamic,
server-rendered list built from the current post’s headings, with a scroll-spy
that marks the section you are reading.

The block render and a render_block_core/post-content filter share one
deterministic slug function, so the table-of-contents anchors always match the
heading ids — regardless of render order or where you place the block (a template
aside or inside the content). Author-provided HTML anchors are respected, and
duplicate headings are de-duplicated (-2, -3).

Two placements: a sidebar rail, or a collapsible disclosure above the
content (a native <details> whose summary can echo the current
section). A “Table of contents (above content)” pattern ships the disclosure
ready to drop before the post content.
Builds from h2–h4 by default (configurable per block).
Unordered list by default; switch to a numbered (ordered) list per block.
Emits the shared toc-list / toc-h2…h4 / is-current class vocabulary so a
theme’s table-of-contents skin couples for free; ships a Material Design 3
token skin (with fallbacks) in the meantime.
Scroll-spy marks the current heading and clears when you leave the content
(above the first heading or past the post content).