perf(nav-panel): optimize workspace session hierarchy indexing and partitioning

src/web-ui/src/app/components/NavPanel/sections/sessions/sessionNavHierarchy.ts

@@ -1,27 +1,50 @@
 import type { Session } from '@/flow_chat/types/flow-chat';
 import { resolveSessionRelationship } from '@/flow_chat/utils/sessionMetadata';
-import { sessionBelongsToWorkspaceNavRow } from '@/flow_chat/utils/sessionOrdering';
+import { sessionOwningWorkspaceId } from '@/flow_chat/utils/sessionOrdering';
+
+/** Fast parent-id resolver: avoid full relationship normalization for ordinary normal sessions. */
+function resolveParentSessionId(session: Session): string | undefined {
+  if (session.sessionKind === 'normal' && !session.parentSessionId && !session.btwOrigin) {
+    return undefined;
+  }
+  return resolveSessionRelationship(session).parentSessionId;
+}
 
 /** Build the same workspace-owned hierarchy for both rows and their count. */
 export function getWorkspaceSessionHierarchy(
   sessions: Iterable<Session>,
   workspaceIds: readonly string[],
 ): { topLevelSessions: Session[]; childrenByParent: Map<string, Session[]> } {
-  const scoped = Array.from(sessions).filter(session => {
-    if (session.isTransient || session.sessionKind === 'subagent') return false;
-    return workspaceIds.length
-      ? workspaceIds.some(workspaceId => sessionBelongsToWorkspaceNavRow(session, workspaceId))
-      : !session.workspaceId;
-  });
-  const knownIds = new Set(scoped.map(session => session.sessionId));
+  const hasWorkspaceScope = workspaceIds.length > 0;
+  const workspaceIdSet = hasWorkspaceScope ? new Set(workspaceIds) : null;
+  const scoped: Session[] = [];
+  const knownIds = new Set<string>();
+
+  for (const session of sessions) {
+    if (session.isTransient || session.sessionKind === 'subagent') continue;
+    const ownerId = sessionOwningWorkspaceId(session);
+    const inScope = hasWorkspaceScope
+      ? (ownerId ? workspaceIdSet!.has(ownerId) : false)
+      : !ownerId;
+    if (!inScope) continue;
+    scoped.push(session);
+    knownIds.add(session.sessionId);
+  }
+
   const topLevelSessions: Session[] = [];
   const childrenByParent = new Map<string, Session[]>();
-  for (const session of scoped) {
-    const parentId = resolveSessionRelationship(session).parentSessionId;
+  // Resolve each parent link once, then bucket children and top-level rows in a
+  // single pass without intermediate arrays.
+  for (let i = 0; i < scoped.length; i++) {
+    const session = scoped[i];
+    const parentId = resolveParentSessionId(session);
     if (parentId && knownIds.has(parentId)) {
-      const children = childrenByParent.get(parentId) ?? [];
-      children.push(session);
-      childrenByParent.set(parentId, children);
+      const children = childrenByParent.get(parentId);
+      if (children) {
+        children.push(session);
+      } else {
+        childrenByParent.set(parentId, [session]);
+      }
     } else {
       topLevelSessions.push(session);
     }