fix(nav): Back restores the focus after a click on the page's blank area

The page region is <main tabIndex=-1>, so a background click focused it and the
Back restore took that for the reader's choice. A resting place is marked and
one predicate (focusIsUnclaimed) decides "the focus is nowhere" everywhere.

frontend/src/lib/history.navPosition.dom.test.tsx

@@ -10,6 +10,7 @@ import { afterEach, describe, expect, it } from "vitest";
 import { act, cleanup, render, waitFor } from "@testing-library/react";
 import { rememberNavPosition } from "./history";
 import { navItemAttrs, useListNav } from "./listNav";
+import { FOCUS_REST_ATTRS } from "./useFocusKeeper";
 
 function List({ id, count, label }: { id: string; count: number; label?: string }) {
   const ref = useRef<HTMLDivElement>(null);
@@ -146,6 +147,22 @@ describe("rememberNavPosition · Back focuses the item a view was left from", ()
   });
 });
 
+// E28 review H1: the page region is `<main tabIndex={-1}>` (the skip link's target), so a mouse click
+// on the page's blank area focuses it. The restore took that for a control the reader had chosen and
+// gave up: Back left the focus on the page instead of on the item the view was opened from.
+describe("rememberNavPosition · the focus resting on the page region", () => {
+  it("still restores when the page region holds the focus", async () => {
+    const { unmount } = render(<Page />);
+    const stamped = leaveFrom("b1", 4);
+    unmount();
+    const region = render(<main tabIndex={-1} data-testid="main" {...FOCUS_REST_ATTRS} />);
+    region.getByTestId("main").focus();
+    landOn(stamped);
+    render(<Page />);
+    await waitFor(() => expect(focused()).toBe("b1"));
+  });
+});
+
 describe("rememberNavPosition · the list it comes back to (deep-review E11 round 2)", () => {
   // Discover: a title opened from "Recently added" and watched — the page's cache expires and
   // "Continue watching" fills with that very title, a rail ABOVE. Found by its place, focus landed in

frontend/src/lib/listNav.ts

@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState, type RefObject } from "react";
 import { dispatch, useKeyScope } from "./keymap";
 import { scrollParent } from "./listPaging";
-import { isStandInFocus } from "./useFocusKeeper";
+import { focusIsUnclaimed } from "./useFocusKeeper";
 
 // ── Keyboard navigation over lists and grids (E11 S4, UX-6) ──────────────────────────────────────
 // Before this, a feed card cost ~10 Tab stops (thumb, play, info, title, channel, five actions), so
@@ -148,6 +148,9 @@ function onFocusIn(e: FocusEvent): void {
   const list =
     item && lists.get(item.parentElement?.closest<HTMLElement>(`[${LIST}]`) as HTMLElement);
   if (item && list) last = { list, index: indexOf(item), el: item };
+  // The focus coming to REST on the page region (a click on its background) is not leaving the list
+  // for other content — like a click on <body>, which fires no focusin at all.
+  else if (focusIsUnclaimed(target)) return;
   // Focus went on to page content that is not an item — the search box, a filter chip: the reader
   // left the list, and whatever it shows next (new results) is not "the slot they were in" (deep-review
   // E11 round 2). A layer opened FROM the item — its menu, a confirm, the Undo toast its removal put
@@ -375,8 +378,7 @@ function refocusSlot(list: NavList, index: number, removed: HTMLElement): void {
       if (++frames < REMOVE_FRAMES) requestAnimationFrame(wait);
       return;
     }
-    const active = document.activeElement;
-    if (active && active !== document.body && !isStandInFocus(active)) return;
+    if (!focusIsUnclaimed()) return;
     const n = list.count();
     if (n > 0) focusIndex(list, Math.min(index, n - 1));
   };
@@ -450,8 +452,7 @@ export function restoreNavPosition(pos: NavPosition): () => void {
   const deadline = Date.now() + RESTORE_MS;
   let timer: ReturnType<typeof setTimeout> | undefined;
   const attempt = () => {
-    const active = document.activeElement;
-    if (active && active !== document.body && !isStandInFocus(active)) return;
+    if (!focusIsUnclaimed()) return;
     const all = ordered();
     const list = pos.label
       ? all.find((l) => labelOf(l) === pos.label)

frontend/src/lib/useFocusKeeper.ts

@@ -41,6 +41,19 @@ export function isStandInFocus(el: Element | null): boolean {
   return el !== null && el === standIn;
 }
 
+/** Marks an element that can hold the focus without being a control: the page's scroll region
+ *  (`<main tabIndex={-1}>`, the skip link's target) takes it from a mouse click on the page's blank
+ *  area. Holding it there is the focus RESTING on the page, the same as <body>. */
+export const FOCUS_REST_ATTRS = { "data-focus-rest": "" } as const;
+
+/** Whether the focus is nowhere the reader chose — <body>, the keeper's stand-in, or a resting place
+ *  (`FOCUS_REST_ATTRS`) — so a restore may take it. The page region used to count as a choice: a
+ *  click on its background focused it, and Back then left the focus there instead of on the item
+ *  the view was opened from (E28 review H1). */
+export function focusIsUnclaimed(el: Element | null = document.activeElement): boolean {
+  return !el || el === document.body || isStandInFocus(el) || el.hasAttribute("data-focus-rest");
+}
+
 /** Where the focus goes once `lost` (removed with the nodes of `record`) has left the page — never
  *  outside `root`: a slot with nothing focusable left (a page swapped for its loading fallback) must
  *  not send the focus to the first control of the whole document. */
@@ -80,8 +93,7 @@ export function useFocusKeeper(ref: RefObject<HTMLElement | null>): void {
       const lost = held;
       if (!lost || lost.isConnected) return;
       held = null;
-      const active = document.activeElement;
-      if (active && active !== document.body) return; // something already took the focus
+      if (!focusIsUnclaimed()) return; // something already took the focus
       for (const r of records) {
         const target = slotFor(r, lost, root);
         if (target) {