The reasoning block memoized its markdown render on [trimmed] alone, so as the
reasoning text streamed in it re-parsed the whole, ever-growing text (marked +
DOMPurify) on every throttled delta (~20Hz) — an O(n^2) CPU storm that pinned the
main thread and froze the chat during a long "thinking" phase. Worse, the block is
collapsed by default, so all that parsing was for a hidden body the user never sees
(html is only shown inside <Collapse in={open}>).
Gate the parse on `open`: collapsed shows the cheap raw-text fallback and does no
markdown parsing; expanding parses the current text once (an instant user click), and
further streaming while open is the normal per-delta append render, like the answer.
Test: assert renderChatMarkdown is not called while collapsed and is called once on
expand.
closes#302
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
agent_coder
2d30ad1fa2