chat: stick-to-bottom via maintainScrollAtEnd on the transcript list

Enable `maintainScrollAtEnd` so the streaming reply keeps the viewport
pinned to the bottom as it grows. Bare = all triggers (dataChange/
itemLayout/layout), so it also follows late row-measure growth and
inset/layout changes back to the end.

Verified on device it coexists with anchoredEndSpace without fighting:
maintainScrollAtEnd takes priority near the end (smooth, no jitter), while
anchoredEndSpace still reserves the reply's initial display area below the
anchored prompt. Moved it out of the "deliberately omitted" list and
refreshed the surrounding scrolling-model comments.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: yyq1025

packages/app/src/components/transcript/chat-panel.tsx

@@ -287,8 +287,10 @@ export function ChatPanel({
         // resting scroll manager (LegendList chat guide).
         freeze={freeze}
         // Reserve blank space below the just-sent user message for its turn so
-        // the streaming response fills it without moving the anchor (fixes the
-        // scroll overshoot). Undefined when idle → normal layout.
+        // the agent's reply has a full initial display area to render into (the
+        // prompt pinned near the top), instead of starting cramped above the
+        // composer. maintainScrollAtEnd then follows the reply's growth to the
+        // bottom. Undefined when idle → normal layout.
         anchoredEndSpace={
           anchorIndex >= 0
             ? { anchorIndex, anchorOffset: headerHeight }
@@ -373,24 +375,20 @@ export function ChatPanel({
         // Without this the last message sits ~insets.bottom too low above
         // composer when the keyboard is shown.
         keyboardOffset={insets.bottom - 8}
-        // ChatGPT-style scrolling, NOT Telegram-style (the upstream
-        // AiChatExample idiom):
+        // Scrolling model:
         //   - initialScrollAtEnd: boot at the latest message on session
         //     re-open. Runs a per-frame rAF ticker that retargets to true
         //     end as items measure and the inset settles (LegendList
         //     retargets the initial scroll after inset changes).
-        //   - During a turn, anchoredEndSpace absorbs streaming growth: the
-        //     reply fills the reserved space below the anchored prompt and
-        //     the viewport NEVER auto-follows. Once the reply outgrows the
-        //     viewport it streams below the fold; the scroll-to-end FAB
-        //     (driven by `isNearEnd`) is the user's way down.
+        //   - maintainScrollAtEnd (above): during a turn the viewport follows
+        //     the streaming reply to the bottom (stick-to-bottom). anchoredEnd-
+        //     Space reserves the reply's initial display area below the just-sent
+        //     prompt (prompt pinned near the top) so it renders into a full
+        //     viewport instead of cramped above the composer; maintainScrollAtEnd
+        //     then follows that growth down. The scroll-to-end FAB (`isNearEnd`)
+        //     is the way back down after the user scrolls up.
         //
         // DELIBERATELY OMITTED:
-        //   - `maintainScrollAtEnd`: auto-follow-the-stream. It defeats the
-        //     anchored pattern the moment the reply outgrows the reserved
-        //     space (the anchor scrolls away mid-read). The upstream AI chat
-        //     example never used it — it belongs to the Telegram-style
-        //     ChatExample only.
         //   - `alignItemsAtEnd`: a Telegram/iMessage idiom (sparse messages
         //     stick to the bottom of the viewport via `flexGrow:1 +
         //     justifyContent:flex-end`). For our ChatGPT layout messages
@@ -401,6 +399,14 @@ export function ChatPanel({
         //     rendered behind the composer backdrop because the alignment
         //     didn't account for `contentInsetEndAdjustment`.
         initialScrollAtEnd
+        // maintainScrollAtEnd: keep the viewport pinned to the bottom as the
+        // streaming reply grows (stick-to-bottom). Bare = true = all triggers
+        // (dataChange/itemLayout/layout) → also follows late row-measure growth
+        // and inset/layout changes back to the end. Internally gated to
+        // near-the-end, so it takes priority there and coexists with
+        // anchoredEndSpace without a fight (verified smooth on device
+        // 2026-06-13: maintainScrollAtEnd wins near the end, no jitter).
+        maintainScrollAtEnd
         // List → UI-thread state mirror (reanimated integration). `isNearEnd`
         // gates the scroll-to-end FAB; no JS re-render involved.
         sharedValues={{ isNearEnd }}