fix(ui): stabilize infinite-stream callbacks + suppress empty-state flash

- useInfiniteStream: wrap fetchNextPage and refresh in useCallback so
  InfiniteScrollArea's IntersectionObserver does not re-subscribe on
  every parent render.
- InfiniteScrollArea: do not render 'End of stream' until at least one
  item has loaded and the initial query has settled (was flashing on
  mount before first fetch).
- AgentHealth: pass isLoading + hasItems to both InfiniteScrollArea
  wrappers.

ui/src/components/InfiniteScrollArea.tsx

@@ -6,6 +6,8 @@ export interface InfiniteScrollAreaProps {
   onTopVisibilityChange?: (atTop: boolean) => void;
   isFetchingNextPage: boolean;
   hasNextPage: boolean;
+  isLoading?: boolean;
+  hasItems?: boolean;
   maxHeight?: number | string;
   children: ReactNode;
   /** Optional caller-owned scroll container ref (e.g. for scroll-to-top on refresh). */
@@ -18,6 +20,8 @@ export function InfiniteScrollArea({
   onTopVisibilityChange,
   isFetchingNextPage,
   hasNextPage,
+  isLoading = false,
+  hasItems = true,
   maxHeight = 360,
   children,
   scrollRef,
@@ -68,7 +72,9 @@ export function InfiniteScrollArea({
       {children}
       <div ref={bottomSentinel} className={styles.sentinel} aria-hidden="true" />
       {isFetchingNextPage && <div className={styles.loadingMore}>Loading more…</div>}
-      {!hasNextPage && <div className={styles.endOfStream}>End of stream</div>}
+      {!hasNextPage && !isLoading && hasItems && (
+        <div className={styles.endOfStream}>End of stream</div>
+      )}
     </div>
   );
 }

ui/src/hooks/useInfiniteStream.ts

@@ -1,5 +1,5 @@
 import { useInfiniteQuery, useQueryClient } from '@tanstack/react-query';
-import { useMemo } from 'react';
+import { useCallback, useMemo } from 'react';
 
 export interface StreamPage<T> {
   data: T[];
@@ -52,14 +52,20 @@ export function useInfiniteStream<T>(args: UseInfiniteStreamArgs<T>): UseInfinit
     [query.data],
   );
 
+  const fetchNextPage = useCallback(() => {
+    if (query.hasNextPage && !query.isFetchingNextPage) query.fetchNextPage();
+  }, [query.hasNextPage, query.isFetchingNextPage, query.fetchNextPage]);
+
+  const refresh = useCallback(() => {
+    queryClient.invalidateQueries({ queryKey: [...queryKey] });
+  }, [queryClient, queryKey]);
+
   return {
     items,
-    fetchNextPage: () => {
-      if (query.hasNextPage && !query.isFetchingNextPage) query.fetchNextPage();
-    },
+    fetchNextPage,
     hasNextPage: !!query.hasNextPage,
     isFetchingNextPage: query.isFetchingNextPage,
     isLoading: query.isLoading,
-    refresh: () => queryClient.invalidateQueries({ queryKey: [...queryKey] }),
+    refresh,
   };
 }

ui/src/pages/AgentHealth/AgentHealth.tsx

@@ -957,6 +957,8 @@ export default function AgentHealth() {
             onTopVisibilityChange={setIsLogAtTop}
             isFetchingNextPage={logStream.isFetchingNextPage}
             hasNextPage={logStream.hasNextPage}
+            isLoading={logStream.isLoading}
+            hasItems={logStream.items.length > 0}
             maxHeight={360}
           >
             {filteredLogs.length > 0 ? (
@@ -993,6 +995,8 @@ export default function AgentHealth() {
             onTopVisibilityChange={setIsTimelineAtTop}
             isFetchingNextPage={eventStream.isFetchingNextPage}
             hasNextPage={eventStream.hasNextPage}
+            isLoading={eventStream.isLoading}
+            hasItems={eventStream.items.length > 0}
             maxHeight={360}
           >
             {feedEvents.length > 0 ? (