test(nav): cover PageScroller save/restore keying (F12)

Renders PageScroller and drives restoreScroll's rAF synchronously with fake
metrics: asserts a forward nav (new entry id) opens at top and Back restores
the offset, and that a new content key within one entry resets to top while
returning restores — the per-(entry,key) contract only the id primitives were
covering before.

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);
+  });
+});