Merge chore/collection-review-tail: shared PlexLibraryPicker + scroll/create tests + 409->nameTaken sweep

backend/tests/test_plex_collections_filter.py

@@ -303,6 +303,54 @@ def test_remove_item_writes_through_to_plex(db, monkeypatch):
         app.dependency_overrides.clear()
 
 
+def _resync_create(db, plex, lib, rk):
+    """Create-path resync stub: the new rating_key has no local row yet, so materialize it."""
+    col = PlexCollection(rating_key=rk, library_id=lib.id, title="Fresh Coll", child_count=1)
+    db.add(col)
+    db.flush()
+    return col
+
+
+def test_create_writes_through_to_plex(db, monkeypatch):
+    _lib, user = _seed_editable(db)
+    stub = _CollStub()
+    client = _admin_client(db, user, monkeypatch, stub, resync=_resync_create)
+    try:
+        r = client.post(
+            "/api/plex/collections",
+            json={"library": "1", "title": "Fresh Coll", "item_rating_key": "m1"},
+        )
+        assert r.status_code == 200, r.text
+        assert "create_collection" in stub.calls
+        body = r.json()
+        assert body["id"] == "NEW"  # the stub's new ratingKey
+        assert body["editable"] is True  # create marks the new collection editable
+    finally:
+        app.dependency_overrides.clear()
+
+
+def test_create_resync_failure_is_422(db, monkeypatch):
+    # The collection was created in Plex (wrote=True) but the follow-up resync read failed → 422 with
+    # the honest "reconcile on the next sync" detail, not a false "write failed".
+    _lib, user = _seed_editable(db)
+    stub = _CollStub()
+
+    def _boom(db_, plex, lib_, rk):
+        raise PlexError("read failed")
+
+    client = _admin_client(db, user, monkeypatch, stub, resync=_boom)
+    try:
+        r = client.post(
+            "/api/plex/collections",
+            json={"library": "1", "title": "Fresh Coll", "item_rating_key": "m1"},
+        )
+        assert r.status_code == 422, r.text
+        assert "reconcile" in r.json()["detail"].lower()
+        assert "create_collection" in stub.calls
+    finally:
+        app.dependency_overrides.clear()
+
+
 def test_mutation_failure_returns_422_write_failed(db, monkeypatch):
     # The Plex mutation itself raised → the write never landed → 422 "Plex write failed", and resync
     # is never reached (stub.calls holds only the mutating call).

frontend/src/components/PageScroller.dom.test.tsx

@@ -0,0 +1,99 @@
+// @vitest-environment jsdom
+//
+// The scroll-restoration contract PageScroller implements: offsets are remembered per
+// (history entry id + content key), so Back to an entry restores where you left it while a forward
+// navigation (a new entry id) or a new content key opens at the top. jsdom lays nothing out, so we
+// feed the <main> fake metrics, drive restoreScroll's rAF loop synchronously, and steer the entry id
+// through window.history.state (what ensureNavId/peekNavId read) — firing the nav bus (notifyNav)
+// stands in for a Back/Forward that doesn't re-render the scroller.
+import { describe, it, expect, vi, beforeAll, beforeEach, afterEach } from "vitest";
+import { render, fireEvent, cleanup } from "@testing-library/react";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return { ...actual, useTranslation: () => ({ t: (k: string) => k }) };
+});
+
+import PageScroller from "./PageScroller";
+import { notifyNav } from "../lib/history";
+
+beforeAll(() => {
+  // useScrollFade wires a ResizeObserver (watch-the-children); jsdom ships none.
+  class Stub {
+    observe() {}
+    unobserve() {}
+    disconnect() {}
+    takeRecords() {
+      return [];
+    }
+  }
+  globalThis.ResizeObserver ??= Stub as unknown as typeof ResizeObserver;
+});
+
+beforeEach(() => {
+  // restoreScroll re-applies the offset over a few rAF frames (the feed needs time to reach height);
+  // run rAF synchronously so a restore lands within the test tick.
+  vi.stubGlobal("requestAnimationFrame", (cb: FrameRequestCallback) => {
+    cb(0);
+    return 0;
+  });
+  vi.stubGlobal("cancelAnimationFrame", () => {});
+});
+
+afterEach(() => {
+  cleanup();
+  vi.unstubAllGlobals();
+});
+
+function setEntry(id: number) {
+  window.history.replaceState({ sfScroll: id }, "");
+}
+
+function sizedMain(container: HTMLElement): HTMLElement {
+  const main = container.querySelector("main")!;
+  // Enough scrollable room for a 300px offset (restoreScroll only sets when scrollHeight-clientHeight
+  // >= the target), and a writable scrollTop jsdom otherwise no-ops.
+  Object.defineProperty(main, "scrollHeight", { configurable: true, value: 1000 });
+  Object.defineProperty(main, "clientHeight", { configurable: true, value: 500 });
+  Object.defineProperty(main, "scrollTop", { configurable: true, writable: true, value: 0 });
+  return main;
+}
+
+describe("PageScroller scroll memory", () => {
+  it("restores per history entry: a forward nav opens at the top, Back restores the offset", () => {
+    setEntry(1);
+    const { container } = render(<PageScroller scrollKey="e:k1">x</PageScroller>);
+    const main = sizedMain(container);
+
+    // The user scrolls entry 1 to 200 — recorded live on scroll.
+    main.scrollTop = 200;
+    fireEvent.scroll(main);
+
+    // Forward navigation → a fresh entry id, same content key → the view opens at the top.
+    setEntry(2);
+    notifyNav();
+    expect(main.scrollTop).toBe(0);
+
+    // Back to entry 1 → its saved offset is restored.
+    setEntry(1);
+    notifyNav();
+    expect(main.scrollTop).toBe(200);
+  });
+
+  it("restores per content key within one entry: a new key resets to top, returning restores", () => {
+    setEntry(10);
+    const { container, rerender } = render(<PageScroller scrollKey="e10:a">x</PageScroller>);
+    const main = sizedMain(container);
+
+    main.scrollTop = 300;
+    fireEvent.scroll(main);
+
+    // A new content key (e.g. a fresh feed filter) in the SAME entry starts at the top.
+    rerender(<PageScroller scrollKey="e10:b">x</PageScroller>);
+    expect(main.scrollTop).toBe(0);
+
+    // Returning to the first key restores its offset.
+    rerender(<PageScroller scrollKey="e10:a">x</PageScroller>);
+    expect(main.scrollTop).toBe(300);
+  });
+});

frontend/src/components/PlexCollectionEditor.tsx

@@ -4,7 +4,7 @@ import { useTranslation } from "react-i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Check, Plus } from "lucide-react";
 import { api } from "../lib/api";
-import { notifyActionError } from "../lib/actionErrors";
+import { notifyCollectionEditError } from "../lib/actionErrors";
 import { useScrollFade } from "../lib/useScrollFade";
 import Modal from "./Modal";
 import { useConfirm } from "./ConfirmProvider";
@@ -102,8 +102,9 @@ export default function PlexCollectionEditor({
       setMembers((prev) => new Set(prev).add(col.id));
       refresh();
     } catch (e) {
-      // Surfaces the 409 when the name duplicates an existing collection (Plex conflates same names).
-      notifyActionError(e, t("plex.collection.editFailed"));
+      // The backend rejects a duplicate name with 409 (Plex conflates same names) — localize it to
+      // nameTaken rather than surfacing the server's raw English detail.
+      notifyCollectionEditError(e, t("plex.collection.editFailed"), t("plex.collection.nameTaken"));
     } finally {
       setBusy(null);
     }

frontend/src/components/PlexCollectionView.tsx

@@ -5,11 +5,11 @@ import { Play, Shuffle, Pencil, Trash2, Plus, X, Lock, Check, Search } from "luc
 import { qk } from "../lib/queryKeys";
 import { api, isIntrinsicReadOnly, type PlexCard, type PlexCollectionDetail } from "../lib/api";
 import { formatRuntime } from "../lib/format";
-import { notifyActionError } from "../lib/actionErrors";
-import { useDebounced } from "../lib/useDebounced";
+import { notifyActionError, notifyCollectionEditError } from "../lib/actionErrors";
 import { useConfirm } from "./ConfirmProvider";
 import { BackBtn, ActionBtn, DetailHero } from "../lib/plexDetailUi";
 import { PlexPosterCard } from "./plexCards";
+import PlexLibraryPicker from "./PlexLibraryPicker";
 import { toggleWatchState } from "../lib/plexWatch";
 import { LoadingState, StateMessage } from "./QueryState";
 
@@ -99,7 +99,8 @@ export default function PlexCollectionView({
       invalidate();
       setRenaming(false);
     } catch (e) {
-      notifyActionError(e, t("plex.collection.editFailed"));
+      // The backend enforces per-library name uniqueness — localize its 409 to nameTaken.
+      notifyCollectionEditError(e, t("plex.collection.editFailed"), t("plex.collection.nameTaken"));
     } finally {
       setBusy(false);
     }
@@ -342,14 +343,7 @@ function AddItemPanel({
 }) {
   const { t } = useTranslation();
   const [term, setTerm] = useState("");
-  const debounced = useDebounced(term.trim(), 300);
   const [added, setAdded] = useState<Set<string>>(new Set());
-  const q = useQuery({
-    queryKey: qk.plexLibrary("collection-add", scope, debounced),
-    queryFn: () => api.plexLibrary({ scope, q: debounced, limit: 12 }),
-    enabled: debounced.length > 0,
-  });
-  const results = q.data?.items ?? [];
 
   async function add(card: PlexCard) {
     try {
@@ -381,43 +375,24 @@ function AddItemPanel({
           <X className="h-4 w-4" />
         </button>
       </div>
-      {debounced.length === 0 ? (
-        <p className="text-muted text-sm">{t("plex.collection.addHint")}</p>
-      ) : q.isLoading ? (
-        <LoadingState label={t("plex.loading")} />
-      ) : q.isError ? (
-        <p className="text-muted text-sm">{t("common.loadError")}</p>
-      ) : results.length === 0 ? (
-        <p className="text-muted text-sm">{t("plex.collection.addNoResults")}</p>
-      ) : (
-        <ul className="flex flex-col gap-1">
-          {results.map((c) => {
-            const inColl = existing.has(c.id) || added.has(c.id);
-            return (
-              <li key={c.id} className="flex items-center gap-3">
-                <img
-                  src={c.thumb}
-                  alt=""
-                  className="h-12 w-8 shrink-0 rounded object-cover"
-                  loading="lazy"
-                />
-                <span className="min-w-0 flex-1 truncate text-sm">
-                  {c.title}
-                  {c.year != null && <span className="text-muted"> · {c.year}</span>}
-                </span>
-                <button
-                  onClick={() => add(c)}
-                  disabled={inColl}
-                  className="glass-card glass-hover inline-flex items-center gap-1 rounded-lg px-2.5 py-1 text-xs disabled:opacity-50"
-                >
-                  {inColl ? <Check className="h-3.5 w-3.5" /> : <Plus className="h-3.5 w-3.5" />}
-                  {inColl ? t("plex.collection.added") : t("plex.collection.add")}
-                </button>
-              </li>
-            );
-          })}
-        </ul>
-      )}
+      <PlexLibraryPicker
+        scope={scope}
+        term={term}
+        emptyHint={<p className="text-muted text-sm">{t("plex.collection.addHint")}</p>}
+        renderAction={(c) => {
+          const inColl = existing.has(c.id) || added.has(c.id);
+          return (
+            <button
+              onClick={() => add(c)}
+              disabled={inColl}
+              className="glass-card glass-hover inline-flex items-center gap-1 rounded-lg px-2.5 py-1 text-xs disabled:opacity-50"
+            >
+              {inColl ? <Check className="h-3.5 w-3.5" /> : <Plus className="h-3.5 w-3.5" />}
+              {inColl ? t("plex.collection.added") : t("plex.collection.add")}
+            </button>
+          );
+        }}
+      />
     </div>
   );
 }

frontend/src/components/PlexLibraryPicker.dom.test.tsx

@@ -0,0 +1,101 @@
+// @vitest-environment jsdom
+//
+// Contract of the shared library search-and-pick list (F6): it owns the debounced query and the
+// full loading/error/empty/no-results/list state machine so the two callers (AddItemPanel in
+// PlexCollectionView, NewCollectionPanel in plexBrowseLists) can't drift. The two things a caller
+// steers — the `enabled` gate (NewCollectionPanel holds it off until the collection name is valid)
+// and the `emptyHint` shown before a term is typed — are the pieces with no test before this file, so
+// they're covered here. i18n is key-passthrough and api/debounce are mocked so the test drives state
+// directly.
+import { type ComponentProps } from "react";
+import { describe, it, expect, afterEach, beforeEach, vi } from "vitest";
+import { render, screen, cleanup, waitFor } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+});
+// Identity debounce so a passed term is reflected immediately.
+vi.mock("../lib/useDebounced", () => ({ useDebounced: (v: unknown) => v }));
+
+// api.plexLibrary stub — returns whatever the current test queued.
+const plexLibrary = vi.fn();
+vi.mock("../lib/api", () => ({ api: { plexLibrary: (...a: unknown[]) => plexLibrary(...a) } }));
+
+import PlexLibraryPicker from "./PlexLibraryPicker";
+
+const card = (id: string, title: string, year = 2000) => ({
+  id,
+  type: "movie" as const,
+  title,
+  year,
+  thumb: `/api/plex/image/${id}`,
+});
+const result = (items: ReturnType<typeof card>[]) => ({
+  scope: "both",
+  total: items.length,
+  offset: 0,
+  limit: 12,
+  items,
+  episodes: [],
+});
+
+beforeEach(() => {
+  plexLibrary.mockReset();
+  plexLibrary.mockResolvedValue(result([]));
+});
+afterEach(cleanup);
+
+const renderPicker = (props: Partial<ComponentProps<typeof PlexLibraryPicker>> = {}) => {
+  const qc = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+  return render(
+    <QueryClientProvider client={qc}>
+      <PlexLibraryPicker
+        scope="both"
+        term=""
+        renderAction={(c) => <button>act:{c.id}</button>}
+        {...props}
+      />
+    </QueryClientProvider>,
+  );
+};
+
+describe("PlexLibraryPicker", () => {
+  it("renders nothing and fires no query while disabled, even with a term", () => {
+    const { container } = renderPicker({ enabled: false, term: "matrix" });
+    expect(container.firstChild).toBeNull();
+    expect(plexLibrary).not.toHaveBeenCalled();
+  });
+
+  it("shows the emptyHint (and fires no query) before a term is typed", () => {
+    renderPicker({ term: "", emptyHint: <p>EMPTY_HINT</p> });
+    expect(screen.getByText("EMPTY_HINT")).toBeTruthy();
+    expect(plexLibrary).not.toHaveBeenCalled();
+  });
+
+  it("queries the library once enabled + a term are both present and renders the rows via renderAction", async () => {
+    plexLibrary.mockResolvedValue(result([card("m1", "The Matrix"), card("m2", "Reloaded")]));
+    renderPicker({ term: "matrix" });
+
+    await waitFor(() => expect(screen.getByText("act:m1")).toBeTruthy());
+    expect(screen.getByText("act:m2")).toBeTruthy();
+    expect(screen.getByText("The Matrix")).toBeTruthy();
+    expect(plexLibrary).toHaveBeenCalledWith({ scope: "both", q: "matrix", limit: 12 });
+  });
+
+  it("renders the honest error state (not an empty/no-results state) when the query fails", async () => {
+    plexLibrary.mockRejectedValue(new Error("boom"));
+    renderPicker({ term: "matrix" });
+
+    await waitFor(() => expect(screen.getByText("common.loadError")).toBeTruthy());
+    expect(screen.queryByText("plex.collection.addNoResults")).toBeNull();
+  });
+
+  it("shows the no-results message when the query returns an empty list", async () => {
+    plexLibrary.mockResolvedValue(result([]));
+    renderPicker({ term: "zzzz" });
+
+    await waitFor(() => expect(screen.getByText("plex.collection.addNoResults")).toBeTruthy());
+  });
+});

frontend/src/components/PlexLibraryPicker.tsx

@@ -0,0 +1,60 @@
+import { type ReactNode } from "react";
+import { useTranslation } from "react-i18next";
+import { useQuery } from "@tanstack/react-query";
+import { qk } from "../lib/queryKeys";
+import { api, type PlexCard } from "../lib/api";
+import { useDebounced } from "../lib/useDebounced";
+import { LoadingState } from "./QueryState";
+
+// A debounced library search-and-pick list, shared by the "Add title" panel (PlexCollectionView) and
+// the "New collection" first-title panel (plexBrowseLists) — both search the library the same way and
+// render the same result rows, differing only in the per-row action. Callers keep their own chrome
+// (search input, name field, close button) and supply `term` + `renderAction`; this owns the query,
+// the loading/error/empty states, and the row markup so those can't drift between the two surfaces.
+export default function PlexLibraryPicker({
+  scope,
+  term,
+  enabled = true,
+  emptyHint = null,
+  renderAction,
+}: {
+  scope: string;
+  /** The raw search term; debounced internally. */
+  term: string;
+  /** Gate the query off (e.g. until a collection name is valid). When false, renders nothing. */
+  enabled?: boolean;
+  /** Shown while the (debounced) term is empty — a hint, or nothing. */
+  emptyHint?: ReactNode;
+  /** The trailing control for each result row (Add / Create-with / …). */
+  renderAction: (card: PlexCard) => ReactNode;
+}) {
+  const { t } = useTranslation();
+  const debounced = useDebounced(term.trim(), 300);
+  const q = useQuery({
+    queryKey: qk.plexLibrary("collection-pick", scope, debounced),
+    queryFn: () => api.plexLibrary({ scope, q: debounced, limit: 12 }),
+    enabled: enabled && debounced.length > 0,
+  });
+  const results = q.data?.items ?? [];
+
+  if (!enabled) return null;
+  if (debounced.length === 0) return <>{emptyHint}</>;
+  if (q.isLoading) return <LoadingState label={t("plex.loading")} />;
+  if (q.isError) return <p className="text-muted text-sm">{t("common.loadError")}</p>;
+  if (results.length === 0) return <p className="text-muted text-sm">{t("plex.collection.addNoResults")}</p>;
+
+  return (
+    <ul className="flex flex-col gap-1">
+      {results.map((c) => (
+        <li key={c.id} className="flex items-center gap-3">
+          <img src={c.thumb} alt="" className="h-12 w-8 shrink-0 rounded object-cover" loading="lazy" />
+          <span className="min-w-0 flex-1 truncate text-sm">
+            {c.title}
+            {c.year != null && <span className="text-muted"> · {c.year}</span>}
+          </span>
+          {renderAction(c)}
+        </li>
+      ))}
+    </ul>
+  );
+}

frontend/src/components/plexBrowseLists.tsx

@@ -4,19 +4,13 @@ import type { TFunction } from "i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Check, ListMusic, Lock, Plus, Search, X } from "lucide-react";
 import { qk } from "../lib/queryKeys";
-import {
-  api,
-  HttpError,
-  isIntrinsicReadOnly,
-  type PlexCard,
-  type PlexCollection,
-} from "../lib/api";
+import { api, isIntrinsicReadOnly, type PlexCard, type PlexCollection } from "../lib/api";
 import { formatRuntime } from "../lib/format";
-import { notifyActionError } from "../lib/actionErrors";
-import { notify } from "../lib/notifications";
+import { notifyCollectionEditError } from "../lib/actionErrors";
 import { useDebounced } from "../lib/useDebounced";
 import { LoadingState, StateMessage } from "./QueryState";
 import PosterTile from "./PosterTile";
+import PlexLibraryPicker from "./PlexLibraryPicker";
 
 // A collection tile's sub-line: item count · total runtime (each dropped when unknown, e.g. show-only
 // collections have no movie runtime).
@@ -86,7 +80,6 @@ function NewCollectionPanel({
   const [name, setName] = useState("");
   const [term, setTerm] = useState("");
   const [busy, setBusy] = useState(false);
-  const debounced = useDebounced(term.trim(), 300);
   const trimmed = name.trim();
   // Advisory only: `existingNames` is the search-scoped, all-library collection union, whereas the
   // backend enforces uniqueness PER library (Plex only conflates same-named collections within one
@@ -94,12 +87,6 @@ function NewCollectionPanel({
   // same-name collection in another library would be un-creatable. The backend's 409 is authoritative.
   const dupHint = trimmed.length > 0 && existingNames.has(trimmed.toLowerCase());
   const nameOk = trimmed.length > 0;
-  const q = useQuery({
-    queryKey: qk.plexLibrary("collection-new", debounced),
-    queryFn: () => api.plexLibrary({ scope: "both", q: debounced, limit: 12 }),
-    enabled: nameOk && debounced.length > 0,
-  });
-  const results = q.data?.items ?? [];
 
   async function create(card: PlexCard) {
     if (!nameOk || busy) return;
@@ -108,13 +95,12 @@ function NewCollectionPanel({
       const col = await api.plexCreateCollection("", trimmed, card.id); // backend derives the library
       onCreated(col.id);
     } catch (e) {
-      // The backend is authoritative on per-library name uniqueness — localize its 409 to nameTaken
-      // (notifyActionError would otherwise surface the server's raw English detail).
-      if (e instanceof HttpError && e.status === 409) {
-        notify({ level: "error", message: t("plex.collection.nameTaken") });
-      } else {
-        notifyActionError(e, t("plex.collection.editFailed"));
-      }
+      // The backend is authoritative on per-library name uniqueness — its 409 is localized to nameTaken.
+      notifyCollectionEditError(
+        e,
+        t("plex.collection.editFailed"),
+        t("plex.collection.nameTaken"),
+      );
       setBusy(false);
     }
   }
@@ -155,38 +141,21 @@ function NewCollectionPanel({
           className="glass-card flex-1 rounded-lg px-3 py-1.5 text-sm focus:outline-hidden focus:ring-2 focus:ring-accent/40 disabled:opacity-50"
         />
       </div>
-      {!nameOk || debounced.length === 0 ? null : q.isLoading ? (
-        <LoadingState label={t("plex.loading")} />
-      ) : q.isError ? (
-        <p className="text-muted text-sm">{t("common.loadError")}</p>
-      ) : results.length === 0 ? (
-        <p className="text-muted text-sm">{t("plex.collection.addNoResults")}</p>
-      ) : (
-        <ul className="flex flex-col gap-1">
-          {results.map((c) => (
-            <li key={c.id} className="flex items-center gap-3">
-              <img
-                src={c.thumb}
-                alt=""
-                className="h-12 w-8 shrink-0 rounded object-cover"
-                loading="lazy"
-              />
-              <span className="min-w-0 flex-1 truncate text-sm">
-                {c.title}
-                {c.year != null && <span className="text-muted"> · {c.year}</span>}
-              </span>
-              <button
-                onClick={() => create(c)}
-                disabled={busy}
-                className="glass-card glass-hover inline-flex items-center gap-1 rounded-lg px-2.5 py-1 text-xs disabled:opacity-50"
-              >
-                <Check className="h-3.5 w-3.5" />
-                {t("plex.collection.createWith")}
-              </button>
-            </li>
-          ))}
-        </ul>
-      )}
+      <PlexLibraryPicker
+        scope="both"
+        term={term}
+        enabled={nameOk}
+        renderAction={(c) => (
+          <button
+            onClick={() => create(c)}
+            disabled={busy}
+            className="glass-card glass-hover inline-flex items-center gap-1 rounded-lg px-2.5 py-1 text-xs disabled:opacity-50"
+          >
+            <Check className="h-3.5 w-3.5" />
+            {t("plex.collection.createWith")}
+          </button>
+        )}
+      />
     </div>
   );
 }

frontend/src/lib/actionErrors.test.ts

@@ -0,0 +1,31 @@
+// @vitest-environment jsdom
+//
+// jsdom (not node): importing ../api pulls in i18n, which reads navigator.language at module load.
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+// Spy on the toast sink so we can assert exactly what each error maps to.
+vi.mock("./notifications", () => ({ notify: vi.fn(() => 0) }));
+
+import { HttpError } from "./api";
+import { notify } from "./notifications";
+import { notifyCollectionEditError } from "./actionErrors";
+
+const notifyMock = vi.mocked(notify);
+beforeEach(() => notifyMock.mockClear());
+
+describe("notifyCollectionEditError", () => {
+  it("maps a backend 409 to the friendly nameTaken message (not the raw server detail)", () => {
+    notifyCollectionEditError(new HttpError(409, "already exists"), "EDIT_FAILED", "NAME_TAKEN");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "NAME_TAKEN" });
+  });
+
+  it("falls back to the server detail for a non-409 HttpError", () => {
+    notifyCollectionEditError(new HttpError(500, "server boom"), "EDIT_FAILED", "NAME_TAKEN");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "server boom" });
+  });
+
+  it("falls back to the generic edit-failed message for a non-HttpError", () => {
+    notifyCollectionEditError(new Error("network"), "EDIT_FAILED", "NAME_TAKEN");
+    expect(notifyMock).toHaveBeenCalledWith({ level: "error", message: "EDIT_FAILED" });
+  });
+});

frontend/src/lib/actionErrors.ts

@@ -12,3 +12,22 @@ export function notifyActionError(err: unknown, fallbackMessage: string): void {
   const detail = err instanceof HttpError ? err.detail : null;
   notify({ level: "error", message: detail || fallbackMessage });
 }
+
+/**
+ * Collection write errors. The backend enforces per-library name uniqueness and returns 409 on a
+ * duplicate; localize that specific case to the friendly `nameTaken` copy (notifyActionError would
+ * otherwise surface the server's raw English detail). Everything else falls back to the generic
+ * edit-failed handling (which still prefers the server's reason when present). Both args are already
+ * translated so this stays i18n-agnostic and unit-testable.
+ */
+export function notifyCollectionEditError(
+  err: unknown,
+  editFailedMessage: string,
+  nameTakenMessage: string,
+): void {
+  if (err instanceof HttpError && err.status === 409) {
+    notify({ level: "error", message: nameTakenMessage });
+    return;
+  }
+  notifyActionError(err, editFailedMessage);
+}