fix(sidebar): keep sticky activity label in sync

Track Chats and Recent section sentinels so the desktop sticky overlay changes identity only when the corresponding section reaches the top of the sidebar.

packages/ui/src/components/session/sidebar/SidebarProjectsList.tsx

@@ -37,6 +37,13 @@ type ProjectSection = {
 const TOP_FADE_MAX_SIZE = 48;
 const TOP_FADE_MIN_SIZE = 32;
 const TOP_FADE_CLEAR_MAX_SIZE = 24;
+type ActivitySectionKey = 'chats' | 'active-now';
+
+const readActivitySectionKey = (element: Element): ActivitySectionKey | null => {
+  const key = element.getAttribute('data-sidebar-activity-sentinel');
+  if (key === 'chats' || key === 'active-now') return key;
+  return null;
+};
 
 const getProjectLabel = (project: ProjectSection['project'], homeDirectory: string | null): string => (
   formatProjectLabel(
@@ -135,6 +142,7 @@ function SidebarProjectsListComponent(props: Props): React.ReactNode {
   // can resolve the scrolling ancestor synchronously (no getComputedStyle
   // walk) and skip the cost of a style recalc on every render.
   const scrollContainerRef = React.useRef<HTMLElement | null>(null);
+  const [leadingActivitySection, setLeadingActivitySection] = React.useState<ActivitySectionKey>('chats');
   // Keep per-scroll measurements out of React state so the interaction guard
   // can read the current fade boundary without rerendering the sidebar.
   const topFadeSizeRef = React.useRef(0);
@@ -167,6 +175,38 @@ function SidebarProjectsListComponent(props: Props): React.ReactNode {
       syncTopFade(scrollContainerRef.current);
     }
   }, [enableStickyFade, hasProjectScroller, syncTopFade]);
+  React.useEffect(() => {
+    const root = scrollContainerRef.current;
+    if (!enableStickyFade || !root || !props.hasSharedSessions) return;
+
+    const sentinels = Array.from(root.querySelectorAll<HTMLElement>('[data-sidebar-activity-sentinel]'));
+    if (sentinels.length === 0) return;
+    const stuckSections = new Set<ActivitySectionKey>();
+    const syncLeadingSection = (): void => {
+      let nextSection = sentinels[0] ? readActivitySectionKey(sentinels[0]) : null;
+      for (const sentinel of sentinels) {
+        const key = readActivitySectionKey(sentinel);
+        if (key && stuckSections.has(key)) nextSection = key;
+      }
+      if (nextSection) setLeadingActivitySection((current) => current === nextSection ? current : nextSection);
+    };
+    const observer = new IntersectionObserver((entries) => {
+      const rootTop = root.getBoundingClientRect().top;
+      for (const entry of entries) {
+        const key = readActivitySectionKey(entry.target);
+        if (!key) continue;
+        if (!entry.isIntersecting && entry.boundingClientRect.top < (entry.rootBounds?.top ?? rootTop)) {
+          stuckSections.add(key);
+        } else {
+          stuckSections.delete(key);
+        }
+      }
+      syncLeadingSection();
+    }, { root, threshold: 0 });
+    sentinels.forEach((sentinel) => observer.observe(sentinel));
+    syncLeadingSection();
+    return () => observer.disconnect();
+  }, [enableStickyFade, props.hasSharedSessions, props.topContent]);
   let stuckProject: ProjectSection['project'] | null = null;
   for (const section of props.projectSections) {
     if (props.stuckProjectHeaders.has(section.project.id)) {
@@ -393,9 +433,11 @@ function SidebarProjectsListComponent(props: Props): React.ReactNode {
             />
           ) : (
             <>
-              <Icon name="history" className="h-3.5 w-3.5 flex-shrink-0 text-muted-foreground/80" />
+              <Icon name={leadingActivitySection === 'chats' ? 'chat-4' : 'history'} className="h-3.5 w-3.5 flex-shrink-0 text-muted-foreground/80" />
               <span className="truncate text-[14px] font-semibold lowercase text-foreground">
-                {t('sessions.sidebar.activity.recentTitle')}
+                {t(leadingActivitySection === 'chats'
+                  ? 'sessions.sidebar.activity.chatsTitle'
+                  : 'sessions.sidebar.activity.recentTitle')}
               </span>
             </>
           )}