The rich markdown editor rebuilt the entire Table-of-Contents outline on
every content change by running a full-document remark parse
(buildMarkdownTableOfContents), then discarded the result whenever the TOC
panel was closed — which is the default state. The parse is driven by the
300ms-debounced serialize path, so it fired ~3x/sec during sustained typing,
with cost scaling linearly with document size.
Gate the memo on showTableOfContents so the parse only runs when the panel is
actually open. Including showTableOfContents in the deps rebuilds the outline
the moment the panel opens, so there is no stale TOC.
Benchmark (config/scripts/markdown-toc-parse-benchmark.mjs), per content
change while typing:
175 KiB doc / 200 headings -> ~65 ms median (13.2 s cumulative over a
~1 min typing burst)
351 KiB doc / 400 headings -> ~138 ms median (27.8 s cumulative)
With the fix this drops to ~0 ms while the panel is closed.
Adds a unit test proving the parse is skipped (and a stable empty-array
reference returned) while closed, and still runs when open.
Co-authored-by: Orca <help@stably.ai>
Neil