fix(ui): back recent header only when stuck

packages/ui/src/components/session/SessionSidebar.tsx

@@ -1705,9 +1705,10 @@ const SessionSidebarComponent: React.FC<SessionSidebarProps> = ({
         openSidebarMenuKey={openSidebarMenuKey}
         expansionState={recentExpandedParents}
         variant="section"
+        isDesktopShellRuntime={isDesktopShellRuntime}
       />
     ) : null,
-    [activitySections, editingId, hasSessionSearchQuery, isVSCode, openSidebarMenuKey, recentExpandedParents, renderSessionNode, showRecentSection],
+    [activitySections, editingId, hasSessionSearchQuery, isDesktopShellRuntime, isVSCode, openSidebarMenuKey, recentExpandedParents, renderSessionNode, showRecentSection],
   );
   const isInlineEditing = Boolean(renamingFolderId || editingId || editingProjectDialogId);
 

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

@@ -10,6 +10,7 @@ import {
   resolveMenuOpenSessionId,
 } from './sessionNodeItemUtils';
 import type { SessionNodeRenderExtras } from './sessionNodeItemUtils';
+import { useStickyHeader } from './hooks/useStickyProjectHeaders';
 
 type ActivityItem = {
   node: SessionNode;
@@ -45,6 +46,7 @@ type Props = {
   variant?: 'section' | 'flat';
   initialVisibleCount?: number;
   batchSize?: number;
+  isDesktopShellRuntime: boolean;
 };
 
 type RenderExtras = SessionNodeRenderExtras;
@@ -60,12 +62,17 @@ export function SidebarActivitySections(props: Props): React.ReactNode {
     variant = 'section',
     initialVisibleCount = MAX_VISIBLE_RECENT_SESSIONS,
     batchSize = MAX_VISIBLE_RECENT_SESSIONS,
+    isDesktopShellRuntime,
   } = props;
   const { t } = useI18n();
   const stickyZoneHeaders = useSessionDisplayStore((state) => state.stickyZoneHeaders);
   const [collapsed, setCollapsed] = React.useState<Set<string>>(new Set());
   const [visibleCountBySection, setVisibleCountBySection] = React.useState<Map<string, number>>(new Map());
   const flatVariant = variant === 'flat';
+  const { isStuck: isRecentHeaderStuck, sentinelRef: recentHeaderSentinelRef } = useStickyHeader({
+    enabled: stickyZoneHeaders && !flatVariant,
+    isDesktopShellRuntime,
+  });
 
   const resetSectionLimit = React.useCallback((key: string) => {
     setVisibleCountBySection((prev) => {
@@ -177,11 +184,17 @@ export function SidebarActivitySections(props: Props): React.ReactNode {
 
         return (
           <div key={section.key} className="relative space-y-1">
-            {/* Zone header styled like a project header band; sticky with a
-                solid sidebar backing so rows never show through. */}
+            {/* Zone header styled like a project header band; its solid
+                backing applies only after the header is actually stuck. */}
+            <div
+              ref={recentHeaderSentinelRef}
+              className="absolute top-0 h-px w-full pointer-events-none"
+              aria-hidden="true"
+            />
             <div className={cn(
               '-ml-2.5 -mr-2',
-              stickyZoneHeaders && 'oc-zone-header-backing sticky top-0 z-20 bg-sidebar',
+              stickyZoneHeaders && 'sticky top-0 z-20 bg-sidebar',
+              stickyZoneHeaders && isRecentHeaderStuck && 'oc-zone-header-backing',
             )}>
               <button
                 type="button"

packages/ui/src/components/session/sidebar/hooks/useStickyProjectHeaders.ts

@@ -7,6 +7,37 @@ type Args = {
   projectHeaderSentinelRefs: React.MutableRefObject<Map<string, HTMLDivElement | null>>;
 };
 
+type StickyHeaderArgs = {
+  enabled: boolean;
+  isDesktopShellRuntime: boolean;
+};
+
+export const useStickyHeader = (args: StickyHeaderArgs) => {
+  const { enabled, isDesktopShellRuntime } = args;
+  const [sentinel, setSentinel] = React.useState<HTMLDivElement | null>(null);
+  const [isStuck, setIsStuck] = React.useState(false);
+
+  const sentinelRef = React.useCallback((node: HTMLDivElement | null) => {
+    setSentinel(node);
+  }, []);
+
+  React.useEffect(() => {
+    if (!enabled || !isDesktopShellRuntime || !sentinel) {
+      setIsStuck(false);
+      return;
+    }
+
+    const observer = new IntersectionObserver(([entry]) => {
+      setIsStuck(entry.intersectionRatio < 1);
+    }, { threshold: 1 });
+
+    observer.observe(sentinel);
+    return () => observer.disconnect();
+  }, [enabled, isDesktopShellRuntime, sentinel]);
+
+  return { isStuck, sentinelRef };
+};
+
 export const useStickyProjectHeaders = (args: Args): Set<string> => {
   const { enabled = true, isDesktopShellRuntime, projectSections, projectHeaderSentinelRefs } = args;
   const [stuckProjectHeaders, setStuckProjectHeaders] = React.useState<Set<string>>(new Set());