chat-ui: memoize timeline rows to stop full re-render per streamed token - #304
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…r token Every streamed inference.text.delta rebuilds the timeline's items array (mergeStreamingReply in chat-workspace.tsx), which re-rendered every MessageParts row in the conversation on every token — not just the growing bubble. Unchanged items keep the same object reference across that rebuild, so a memo guard that ignores handler props (recreated every render regardless, carry no displayed data) skips the re-render for every row whose data didn't actually change. CL-6625
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Summary
CL-6625 (chat feels slow/jerky) found the top offender:
WorkbenchTimeline(packages/chat-ui/src/timeline.tsx) has noReact.memoanywhere, and theitemsarray it receives is rebuilt fresh on every render (mergeStreamingReply/mergePendingSends/appendReplyTimedOutNotice, chat-workspace.tsx:1276-1285). A real reply firesinference.text.deltamany times per second (see streaming-reply.ts's own docs), and each one re-renders everyMessagePartsrow in the whole conversation — not just the growing streaming bubble.Unchanged items keep their own object reference across that rebuild (the merge helpers spread
[...items, newItem]without touching existing elements), so a memo comparator that treats handler props as always-equal (they're recreated every render regardless and carry no displayed data, only callbacks) makes memoization actually effective: rows whose data hasn't changed skip re-render entirely during a live turn.This is the top-ranked fix from the CL-6625 findings comment — smallest safe change, no transport/streaming-architecture rework, and stays out of
failed-turn-strip/the composer retry path (CL-6624's territory).Test plan
tsc -p packages/chat-ui/tsconfig.json --noEmiteslint packages/chat-ui/src/timeline.tsxbun run testinpackages/chat-ui(665 pass)