fix(chat): stop double scroll write on prepend that resonates into oscillation

When older history is prepended while the viewport is pinned to the bottom, the
timeline controller wrote the re-pin manually (scrollTop += delta). That write
is not flagged as programmatic, so useChatAutoFollow's scroll handler treated it
as movement and issued its own correcting scroll — a redundant up/down move on
every prepend. On most setups it settles after one move, but with different
virtualizer measurement/timing it never converges, producing the reported
infinite up/down scroll glitch.

When pinned, delegate the prepend re-pin to auto-follow's goToBottom('instant'):
a single authoritative write to the bottom that IS marked programmatic, so
auto-follow ignores it instead of fighting it. The released case (user reading
back through history) is unchanged and still preserves the read position.

This also covers the on-open history auto-load (loadEarlierIfPinnedViewport-
Underfilled), which only runs while pinned, so its prepends now go through the
single writer too.

packages/ui/src/components/chat/hooks/useChatTimelineController.ts

@@ -350,6 +350,25 @@ export const useChatTimelineController = ({
         const container = scrollRef.current;
         if (!container) return;
 
+        // Bottom-pinned: auto-follow is the single owner of the scroll position.
+        // Route the prepend re-pin through goToBottom (a programmatic, authoritative
+        // instant write to the bottom) rather than a manual scrollTop adjustment. A
+        // manual write here is NOT marked programmatic, so auto-follow's scroll
+        // handler treats it as movement and issues its own correcting scroll — a
+        // redundant up/down move on every prepend that, on some setups, resonates
+        // into the reported infinite oscillation. Delegating keeps exactly one
+        // writer and no fight.
+        if (isPinnedRef.current) {
+            prePrependScrollRef.current = null;
+            goToBottom('instant');
+            prependTrackingRef.current = {
+                oldestId: renderedMessages[0]?.info?.id ?? null,
+                newestId: renderedMessages[renderedMessages.length - 1]?.info?.id ?? null,
+                scrollHeight: container.scrollHeight,
+            };
+            return;
+        }
+
         const snap = prePrependScrollRef.current;
         if (snap) {
             prePrependScrollRef.current = null;
@@ -395,7 +414,7 @@ export const useChatTimelineController = ({
             newestId: renderedMessages[renderedMessages.length - 1]?.info?.id ?? null,
             scrollHeight: container.scrollHeight,
         };
-    }, [renderedMessages, scrollRef, restoreViewportAnchor]);
+    }, [renderedMessages, scrollRef, restoreViewportAnchor, goToBottom]);
 
     const revealBufferedTurns = React.useCallback(async (): Promise<boolean> => false, []);