Fix scroll behavior when switching between chats (dyad-sh#2643)

## Summary
- Fix scroll behavior when switching between chats with existing
messages
- Use double `requestAnimationFrame` to wait for Virtuoso to render
before scrolling to bottom when switching chats
- Distinguish between chat switches and new message sends to handle
scrolling appropriately
- Avoid premature scrolling when switching to chats where messages
haven't been fetched yet

## Test plan
1. Open Dyad and start a chat with some messages
2. Start another chat with messages
3. Switch between chats and verify scroll position stays at the bottom
4. Send a new message and verify it scrolls to show the new message

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixes chat auto-scroll to keep the view anchored at the bottom when
switching chats or sending messages. Prevents flicker and jumps before
messages render.

- **Bug Fixes**
- Distinguish chat switches vs new sends; adjust scroll timing
accordingly.
- On chat switch with existing messages, wait for Virtuoso to render
(double requestAnimationFrame), then scroll to bottom instantly.
- On new message send, wait for the placeholder to render (double
requestAnimationFrame) before scrolling; skip auto-scroll when switching
to a chat with no messages yet.

<sup>Written for commit 8814a60.
Summary will update on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Scoped to `ChatPanel` scroll timing logic; risk is limited to possible
UI regressions (missed/extra scroll) when switching chats or starting
streams.
> 
> **Overview**
> Fixes `ChatPanel` auto-scroll behavior by **distinguishing chat
switches from new stream starts** and delaying the scroll until after
Virtuoso has rendered.
> 
> On chat switch, it now scrolls to bottom *only after messages exist*
(avoiding premature scroll before fetch/render) and uses a double
`requestAnimationFrame` with `instant` scrolling; on new message sends
(`streamCount` changes), it similarly waits for render before scrolling
with the default behavior.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
8814a60. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: 2 people

src/components/ChatPanel.tsx

@@ -79,11 +79,38 @@ export function ChatPanel({
 
   // Scroll to bottom when a new stream starts (user sent a message)
   const streamCount = chatId ? (streamCountById.get(chatId) ?? 0) : 0;
+  const messages = chatId ? (messagesById.get(chatId) ?? []) : [];
+
+  // Track previous chatId to detect chat switches
+  const prevChatIdRef = useRef<number | undefined>(undefined);
+
   useEffect(() => {
+    const isChatSwitch = prevChatIdRef.current !== chatId;
+    prevChatIdRef.current = chatId;
+
     isAtBottomRef.current = true;
     setShowScrollButton(false);
-    scrollToBottom();
-  }, [chatId, streamCount, scrollToBottom]);
+
+    if (isChatSwitch && messages.length > 0) {
+      // When switching chats with existing messages, wait for Virtuoso to render
+      // then scroll to ensure we're at the bottom
+      requestAnimationFrame(() => {
+        requestAnimationFrame(() => {
+          scrollToBottom("instant");
+        });
+      });
+    } else if (!isChatSwitch) {
+      // For stream count changes (new message sent), wait for Virtuoso to render
+      // the placeholder message before scrolling
+      requestAnimationFrame(() => {
+        requestAnimationFrame(() => {
+          scrollToBottom();
+        });
+      });
+    }
+    // Note: if isChatSwitch && messages.length === 0, we don't scroll yet.
+    // The messages will be fetched and this effect will re-run with messages.length > 0.
+  }, [chatId, streamCount, messages.length, scrollToBottom]);
 
   const fetchChatMessages = useCallback(async () => {
     if (!chatId) {
@@ -102,7 +129,6 @@ export function ChatPanel({
     fetchChatMessages();
   }, [fetchChatMessages]);
 
-  const messages = chatId ? (messagesById.get(chatId) ?? []) : [];
   const isStreaming = chatId ? (isStreamingById.get(chatId) ?? false) : false;
 
   // Scroll to bottom when streaming completes to ensure footer content is visible,