💬 fix: Show a Thought on the Live Header Only as Finished Sentences

[danny-avila]

Summary

While a span is only reasoning, the live header previews the thought one sentence at a time, and it previewed the sentence the model was still writing, word by word. On screen that is a run of fragments ("This is a simple greeting, not", "I should respond in a friendly,") that change before they can be read and are replaced mid-thought by the next fragment, which reads as disjointed.

The header now shows the last sentence that ended, never the one in progress. A finished sentence holds the line for at least a second before the next finished one may replace it, and until the first sentence ends the header keeps its generic line ("Thinking…" or the reasoning label). A Latin sentence mark closes a sentence only before whitespace or the end of the text, so "3.5" and "v2.1" stay inside one sentence; CJK marks need no space, as before.

The measurement that let short sentences stream unthrottled until they filled the row (a ResizeObserver plus a scroll-width read per paint) is removed with it: the line no longer changes per word, so there is nothing to meter.

Stacked on #16391 and targets its branch.

How it works

// live.ts
const boundary = /[.!?](?:\s+|$)|[。！？]\s*/g;
// walk every boundary in the bounded tail; `complete` is the text from the
// previous boundary to the newest mark; the remainder after it is ignored.

LivePhaseHeader throttles a think: line at LIVE_REASONING_HOLD_MS (1000ms) and everything else at LIVE_ACTIVITY_THROTTLE_MS (500ms). The throttle is leading-edge, so a sentence that ends a second or more after the last paint shows at once; only sentences that end faster than that wait.

Type of change

• Bug fix

Testing

Automated tests:

• LiveParity.test.tsx: new "shows a thought only as finished sentences, each held for a second" and "keeps decimals and versions inside one sentence"; the CJK preview case and the bounded-window case updated to the finished sentence; the two fill-measurement cases removed with the mechanism
• ContentParts.integration.test.tsx: the one-sentence-at-a-time case now waits for the second sentence to end
• cd client && npx jest LiveParity ActivityPhaseGroup ContentParts — 180 tests
• npx tsc --noEmit in client: no errors in touched files

Before / after screenshot

Captured while the first reasoning sentence is still streaming. Before, the header exposes the in-progress fragment “First I”; after, it keeps “Thinking...” until a sentence ends.

349e01f3a3cd (before) → 118bbcf2e37c (after).