Preserve thread selection during background updates

web/src/lib/sidebar-selection.ts

@@ -0,0 +1,31 @@
+export type SidebarSelection = { kind: "chat" | "search"; id: string };
+
+type WorkspaceSelectionItem = { type: SidebarSelection["kind"]; id: string };
+
+type ResolveSidebarSelectionOptions = {
+  initialSelection?: SidebarSelection;
+  selectFallback?: boolean;
+};
+
+export function resolveSidebarSelectionAfterRefresh(
+  current: SidebarSelection | null,
+  workspaceItems: WorkspaceSelectionItem[],
+  { initialSelection, selectFallback = false }: ResolveSidebarSelectionOptions = {}
+): SidebarSelection | null {
+  const hasItem = (candidate: SidebarSelection | null | undefined) => {
+    if (!candidate) return false;
+    return workspaceItems.some((item) => item.type === candidate.kind && item.id === candidate.id);
+  };
+
+  if (hasItem(current)) {
+    return current;
+  }
+  if (hasItem(initialSelection)) {
+    return initialSelection ?? null;
+  }
+  if (!selectFallback) {
+    return null;
+  }
+  const first = workspaceItems[0];
+  return first ? { kind: first.type, id: first.id } : null;
+}