The expanded "Thinking" block re-ran marked+DOMPurify and re-set
dangerouslySetInnerHTML with the whole growing reasoning text on every
throttled stream delta (~20 Hz) — the O(n²) hole #302 deliberately left
open ("expanded while streaming"). In Safari this saturates the main
thread and freezes the entire tab during long agent runs, including
while the window is minimized (the JS storm keeps running) and on
re-expanding it mid-turn (one huge layout burst).
- streaming-plain-text.tsx (new): chunked plain-text renderer; chunks
split at blank-line boundaries with an append-only stable-prefix
invariant, so per delta only the tail chunk's text node updates —
no marked, no DOMPurify, no innerHTML swaps.
- reasoning-block.tsx: parse markdown only when expanded AND finalized
(one-time); while streaming, render chunked plain text; collapsed
stays parse-free (#302 unchanged).
- message-item.tsx / message-list.tsx: reasoning liveness = part
state:"streaming" AND the turn is live AND the row is the tail —
a part stranded at state:"streaming" (manual Stop during thinking,
or a provider that never emits reasoning-end) finalizes at turn end
and never re-activates when later turns stream.
Verified with the Chrome perf harness: per-delta marked/DOMPurify work
is gone from the hot path; collapsed streaming stays at 0 long tasks
up to 143k tokens even at 4x CPU throttle; finalized expanded blocks
still render parsed markdown. 245 client tests green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
agent_vscode
351860ba4b