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/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: {