feat(chat): block-level streaming reveal with a gliding follow

Token-by-token streaming mutates the trailing paragraph in place on
every tick: words rewrap, the last line jitters, and the whole reply
reads as flicker. Streamed text now commits only up to the last complete
line — prose arrives a paragraph at a time (a markdown paragraph is one
logical line), code fences reveal line by line, tables row by row — and
a shown block never changes again. A paragraph that runs long without a
newline releases at the last sentence (then word) boundary so the stream
never stalls. Applies to assistant text and reasoning; tool output keeps
its raw tail.

With growth arriving in block steps, the end follow switches to the
list's animated mode so each step is a glide — reveal and scroll read as
one continuous motion. The gesture opt-out now measures at-end from the
live list state instead of the cached flag, which the animated glide
deliberately leaves stale while trailing the edge; without that, a drag
during a glide could leave the scroll-to-bottom pill unshown.

Measured: end-following holds at distance 0 for the whole stream, and
the mobile drag opt-out shows the pill in three of three runs. The
continuous glide costs ~15% more main-thread time per streamed character
than the instant follow — the price of the motion.

packages/ui/src/components/chat/MessageList.tsx

@@ -1071,7 +1071,10 @@ const TimelineList = React.memo(({
                 // resize settles.
                 maintainScrollAtEnd={anchoredEndSpace || !streamingAutoFollowEnabled || isWidthResizing || endPinningReleased
                     ? false
-                    : { animated: false, on: { dataChange: true, itemLayout: true, layout: true, footerLayout: true } }}
+                    // Animated: with block-level reveal the content grows in
+                    // paragraph/line steps, and the animated follow turns each
+                    // step into a glide — reveal and scroll read as one motion.
+                    : { animated: true, on: { dataChange: true, itemLayout: true, layout: true, footerLayout: true } }}
                 // Prepending older history must not move what the user is
                 // reading. Size restoration applies only during a width
                 // resize — see the observer above.

packages/ui/src/components/chat/lib/streamTextCommit.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, test } from 'bun:test';
+
+import { commitStreamedText } from './streamTextCommit';
+
+describe('commitStreamedText', () => {
+    test('holds an incomplete short paragraph entirely', () => {
+        expect(commitStreamedText('An unfinished thought abo')).toBe('');
+    });
+
+    test('commits up to the last complete line', () => {
+        expect(commitStreamedText('First paragraph.\n\nSecond par')).toBe('First paragraph.\n\n');
+    });
+
+    test('reveals code fences line by line', () => {
+        const text = '```py\nprint("a")\nprint("b';
+        expect(commitStreamedText(text)).toBe('```py\nprint("a")\n');
+    });
+
+    test('releases a long held paragraph at the last sentence boundary', () => {
+        const sentence = 'A finished sentence lives here. ';
+        const text = sentence.repeat(12) + 'and an unfinished trail';
+        expect(commitStreamedText(text)).toBe(sentence.repeat(12));
+    });
+
+    test('falls back to the last word boundary without sentences', () => {
+        const words = 'word '.repeat(70);
+        const text = words + 'unfinishe';
+        expect(commitStreamedText(text)).toBe(words);
+    });
+
+    test('keeps unbreakable runs intact rather than splitting them', () => {
+        const run = 'x'.repeat(400);
+        expect(commitStreamedText(run)).toBe(run);
+    });
+
+    test('empty input stays empty', () => {
+        expect(commitStreamedText('')).toBe('');
+    });
+});

packages/ui/src/components/chat/lib/streamTextCommit.ts

@@ -0,0 +1,47 @@
+// Block-level streaming reveal.
+//
+// Token-by-token streaming mutates the trailing paragraph in place on every
+// tick: words rewrap, the last line jitters, and the reader's eye fights the
+// motion. Committing only up to the last COMPLETE line keeps every rendered
+// block immutable once it appears — prose arrives a paragraph at a time (a
+// markdown paragraph is one logical line), code fences reveal line by line,
+// tables row by row — and the only remaining motion is the follow scroll.
+//
+// A paragraph with no newline for a long stretch must not stall the stream,
+// so once the held tail outgrows a threshold it is committed at the last
+// sentence boundary (falling back to the last word boundary).
+
+const HOLD_MAX_CHARS = 320;
+
+const SENTENCE_END = /[.!?…][)"'»”’]?\s/g;
+
+export const commitStreamedText = (text: string): string => {
+    if (text.length === 0) return text;
+
+    const lastNewline = text.lastIndexOf('\n');
+    const committed = lastNewline === -1 ? '' : text.slice(0, lastNewline + 1);
+    const held = text.slice(committed.length);
+
+    if (held.length <= HOLD_MAX_CHARS) {
+        return committed;
+    }
+
+    // The held paragraph got long: release it up to the last finished
+    // sentence so the block still never mutates mid-sentence.
+    let lastSentenceEnd = -1;
+    for (const match of held.matchAll(SENTENCE_END)) {
+        lastSentenceEnd = match.index + match[0].length;
+    }
+    if (lastSentenceEnd > 0) {
+        return committed + held.slice(0, lastSentenceEnd);
+    }
+
+    // No sentence boundary either (a URL, a very long token run): release up
+    // to the last word boundary, keeping only the incomplete word held.
+    const lastSpace = held.lastIndexOf(' ');
+    if (lastSpace > 0) {
+        return committed + held.slice(0, lastSpace + 1);
+    }
+
+    return text;
+};

packages/ui/src/components/chat/message/parts/ReasoningPart.tsx

@@ -9,6 +9,7 @@ import { useI18n } from '@/lib/i18n';
 import { useUIStore } from '@/stores/useUIStore';
 import { MarkdownRenderer } from '../../MarkdownRenderer';
 import { useStreamingTextThrottle } from '../../hooks/useStreamingTextThrottle';
+import { commitStreamedText } from '../../lib/streamTextCommit';
 import type { StreamPhase } from '../types';
 
 const TOOL_ROW_TEXT_CLASS = '!text-[length:var(--text-meta)] !leading-5 sm:!leading-6 tracking-normal';
@@ -431,11 +432,14 @@ const ReasoningPart = React.memo(({
     const time = partWithText.time;
     const canBeStreaming = streamPhase === undefined || streamPhase !== 'completed';
     const isStreaming = chatRenderMode === 'live' && canBeStreaming && typeof time?.end !== 'number';
-    const throttledText = useStreamingTextThrottle({
+    const throttledTextRaw = useStreamingTextThrottle({
         text: textContent,
         isStreaming,
         identityKey: `${messageId}:${part.id ?? 'reasoning'}`,
     });
+    // Same block-level reveal as assistant text: a shown reasoning paragraph
+    // never mutates in place.
+    const throttledText = isStreaming ? commitStreamedText(throttledTextRaw) : throttledTextRaw;
 
     // Show reasoning even if time.end isn't set yet (during streaming)
     // Only hide if there's no text content

packages/ui/src/components/chat/message/parts/assistantTextVisibility.ts

@@ -1,9 +1,16 @@
+import { commitStreamedText } from '../../lib/streamTextCommit';
+
 export const resolveAssistantDisplayText = (input: {
     textContent: string;
     throttledTextContent: string;
     isStreaming: boolean;
 }): string => {
-    return input.isStreaming ? input.throttledTextContent : input.textContent;
+    // While streaming, reveal whole blocks only: rendering stops at the last
+    // complete line so a shown paragraph never mutates in place. The held
+    // tail lands with the next line break (or the finalize pass).
+    return input.isStreaming
+        ? commitStreamedText(input.throttledTextContent)
+        : input.textContent;
 };
 
 export const shouldRenderAssistantText = (input: {

packages/ui/src/hooks/useChatTimelineScroll.ts

@@ -7,6 +7,7 @@ import { useUIStore } from '@/stores/useUIStore';
 import {
     CHAT_LIST_ANCHOR_OFFSET,
     getAnchoredTurnMetrics,
+    resolveTimelineIsAtEnd,
     type TimelineListMeasurementState,
     type TimelineScrollMode,
 } from '@/components/chat/lib/scroll/timelineScrollAnchoring';
@@ -204,9 +205,14 @@ export const useChatTimelineScroll = ({
         liveFollowGenerationRef.current = null;
         setUserOwnsScroll(true);
         // The end may already have been left by our own movement, in which
-        // case no further at-end transition will fire. This is an explicit
-        // gesture — show the pill immediately, no debounce.
-        if (!isAtEndRef.current) {
+        // case no further at-end transition will fire — and while an animated
+        // follow glide trails the live edge, isAtEndRef is deliberately not
+        // updated, so measure the real distance instead of trusting it. This
+        // is an explicit gesture — show the pill immediately, no debounce.
+        const listState = listRef.current?.getState();
+        const atEndNow = (listState ? resolveTimelineIsAtEnd(listState) : undefined) ?? isAtEndRef.current;
+        isAtEndRef.current = atEndNow;
+        if (!atEndNow) {
             cancelShowButtonTimer();
             setShowScrollButton(true);
         }