fix: Reset scroll state when switching between split/tabbed view

The "Back to bottom" button was incorrectly appearing after switching
from split view to tabbed view because scroll-related refs weren't
being reset when the session changed.

Added session change detection that resets:
- userScrolledAway ref
- showScrollButton state
- newMessageCount counter
- processedMessages cache

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/MessageList.jsx

@@ -182,6 +182,42 @@ export const MessageList = memo(function MessageList({ messages, isProcessing, o
   const [newMessageCount, setNewMessageCount] = useState(0);
   const prevMessageCount = useRef(messages.length);
   const userScrolledAway = useRef(false);
+  const prevHostId = useRef(hostId);
+
+  // Cache for incremental updates - avoid full rebuild on streaming content changes
+  // Moved up so it's available for the reset effect
+  const processedCacheRef = useRef({ messages: [], result: [], toolResultMap: new Map() });
+
+  // Reset scroll state when switching sessions (hostId changes or messages array replaced)
+  // This fixes the "scroll to bottom" button appearing incorrectly after switching from split view
+  useLayoutEffect(() => {
+    // Detect session change by checking if hostId changed or if messages were reset
+    const hostChanged = prevHostId.current !== hostId;
+    const messagesReset = messages.length === 0 ||
+      (prevMessageCount.current > 0 && messages.length > 0 &&
+       messages[0]?.timestamp !== processedCacheRef.current.messages[0]?.timestamp);
+
+    if (hostChanged || messagesReset) {
+      // Reset all scroll-related state
+      userScrolledAway.current = false;
+      setShowScrollButton(false);
+      setNewMessageCount(0);
+      prevMessageCount.current = messages.length;
+      prevHostId.current = hostId;
+
+      // Clear processed cache to force rebuild
+      processedCacheRef.current = { messages: [], result: [], toolResultMap: new Map() };
+
+      // Scroll to bottom after session switch
+      if (containerRef.current) {
+        requestAnimationFrame(() => {
+          if (containerRef.current) {
+            containerRef.current.scrollTop = containerRef.current.scrollHeight;
+          }
+        });
+      }
+    }
+  }, [hostId, messages]);
 
   // Check if scrolled to bottom
   const checkIfAtBottom = useCallback(() => {
@@ -203,9 +239,6 @@ export const MessageList = memo(function MessageList({ messages, isProcessing, o
     }
   }, [checkIfAtBottom]);
 
-  // Cache for incremental updates - avoid full rebuild on streaming content changes
-  const processedCacheRef = useRef({ messages: [], result: [], toolResultMap: new Map() });
-
   // Preprocess messages to pair tool_use with tool_result
   // Optimized: only rebuild when structure changes, not content
   const processedMessages = useMemo(() => {