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>
The floating AI chat rendered NOTHING for the assistant turn (user bubble +
"thinking" dots showed, but the streamed text and tool-call cards never
appeared) even though the agent ran server-side. The parts DID arrive in
`useChat.messages` — this was purely a render freeze.
Root cause: the MessageItem `React.memo` comparator (#182) decided whether to
re-render by recomputing `messageSignature(prev.message)` vs
`messageSignature(next.message)` inside `arePropsEqual` (plus a
`prev.message === next.message` fast path). But the AI SDK (ai@6 /
@ai-sdk/react@3) streams a turn by MUTATING the same `parts` in place and
handing back a message wrapper that SHARES those mutated parts. So inside the
comparator both `prev.message` and `next.message` already reflect the latest
content — the two signatures are ALWAYS equal — and the memo skipped every
post-mount render. The assistant row therefore froze at its initial empty
(null) render; reasoning-first providers (e.g. z.ai/GLM) start with a
non-visible reasoning part, so the whole answer + tool cards never showed.
Fix: snapshot the signature in the PARENT (MessageList) at render time and pass
it to MessageItem as an immutable `signature` string prop; `arePropsEqual` now
compares that prop. A captured string is immutable, so `prev.signature` holds
the previous render's content and `next.signature` the new content — they differ
as the turn streams in and the row re-renders. Drop the now-incorrect
`prev.message === next.message` fast path (same-ref-but-mutated must still
re-render). MarkdownPart's per-part memo is unaffected (it already keys on the
primitive `text`).
Verified end-to-end against a real OpenAI-compatible provider: the assistant
turn (reasoning + streamed text + tool-call card) now renders live and on
finish. Regression tests added (render + comparator) that fail before / pass
after.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Address non-blocking review items on the AI-chat stream-perf PR:
- Drop the unused `metadata` param from the `msg` test factory in
message-item.test.ts; no caller passed it.
- Add a per-part-kind coupling guard to message-signature.test.ts that, for
each part kind rendered today (text, reasoning, tool-*) plus the metadata
banners, asserts that mutating a field the MessageItem render body DRAWS
flips messageSignature — an executable lock for the load-bearing memo
invariant documented in message-signature.ts.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Resolve the PR #182 code-review (Request changes) on top of the already-merged
develop (the merge commit preserves both the markdown useMemo and the
collapseBlankLines fix in reasoning-block.tsx).
- Extract messageSignature from message-item.tsx into utils/message-signature.ts
(matches the feature's "pure UIMessage helper + colocated test" convention) and
export arePropsEqual so the memo seam is unit-testable. No logic change.
- Add utils/message-signature.test.ts covering every change signal (text grows,
part appended, state flip, output appears, errorText appears, usage.reasoningTokens
arriving on finish-step, metadata error/finishReason) plus the negative
content-identical-clone case.
- Add components/message-item.test.ts for arePropsEqual (each prop diff -> false,
identity fast-path -> true, same-content-different-object -> true, changed -> false).
- Add components/message-item-memo.test.tsx: render-level proof that finalized text
parts are not re-parsed when only a tail part grows (MarkdownPart memo).
- CHANGELOG: add the user-facing 100% CPU freeze fix under [Unreleased] / Fixed.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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