Interesting approach. When I was going to tackle this one I thought it might be interesting to make it static—it'd update whenever you interact with the editor, and would save HTML directly. That would be regenerated whenever you edited headings in the editor.

Yah, I thought about that - having to write duplicate code (one for the block render in JS, one for the frontend render in PHP) isn't ideal, but it means that any headers added by not-Gutenberg will be correctly rendered.

I tend to lean towards implementing this kind of logic in the server side, instead of requiring every editor that interacts with a WordPress site to implement it.

Codecov Report

Merging #2871 into master will decrease coverage by <.01%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #2871      +/-   ##
==========================================
- Coverage   34.07%   34.07%   -0.01%     
==========================================
  Files         192      193       +1     
  Lines        5675     5681       +6     
  Branches      996      997       +1     
==========================================
+ Hits         1934     1936       +2     
- Misses       3165     3169       +4     
  Partials      576      576

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In order to show the ToC in the block render inside Gutenberg, I believe I'm blocked on #2473.

I think you are no longer blocked. It is possible to access every piece of the state through data module as described here: https://github.com/WordPress/gutenberg/tree/master/data.

@pento, feel free to reopen this one. For now, with the branch stale, I'll close it as we're making sense of all open PRs. 🙇