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/components/DiagnosticsSettings.tsx

@@ -4,6 +4,7 @@ import { useTranslation } from "react-i18next";
 
 import { api, type DiagnosticsLevel } from "../lib/api";
 import { levelInForce } from "../lib/diagnostics";
+import { focusIsUnclaimed } from "../lib/useFocusKeeper";
 import { updateMeCache, useMe } from "../lib/useMe";
 import { Button } from "./ui/Button";
 import InlineNotice from "./ui/InlineNotice";
@@ -61,7 +62,7 @@ export default function DiagnosticsSettings() {
     const from = closedFrom.current;
     if (!from || from.isConnected) return;
     closedFrom.current = null;
-    if (document.activeElement && document.activeElement !== document.body) return;
+    if (!focusIsUnclaimed()) return;
     pickerRef.current?.querySelector<HTMLElement>('[aria-pressed="true"]')?.focus();
   });
   const watchFocus = () => {

frontend/src/components/IssueComposer.tsx

@@ -19,6 +19,7 @@ import {
 import { moduleDef } from "../lib/modules";
 import type { Page } from "../lib/urlState";
 import { qk } from "../lib/queryKeys";
+import { focusIsUnclaimed } from "../lib/useFocusKeeper";
 import { LEVEL_TAG, logClock } from "./logLineParts";
 import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
@@ -54,7 +55,7 @@ export default function IssueComposer({ id }: { id: string }) {
   useEffect(
     () => () => {
       window.setTimeout(() => {
-        if (document.activeElement === document.body)
+        if (focusIsUnclaimed())
           document.querySelector<HTMLElement>('nav [aria-current="page"]')?.focus();
       }, 0);
     },

frontend/src/components/ui/PageScroller.tsx

@@ -1,6 +1,6 @@
 import { useCallback, useLayoutEffect, useRef, useState, type ReactNode } from "react";
 import { useScrollFade } from "../../lib/useScrollFade";
-import { useFocusKeeper } from "../../lib/useFocusKeeper";
+import { FOCUS_REST_ATTRS, useFocusKeeper } from "../../lib/useFocusKeeper";
 import { StickyTopProvider } from "./stickyTop";
 import { CONTENT_ID } from "./landmarks";
 import { ensureNavId, peekNavId, subscribeNav } from "../../lib/history";
@@ -158,11 +158,13 @@ export default function PageScroller({
 
   return (
     // `id` + `tabIndex={-1}`: the skip link's target (ui/SkipLink) takes the focus, so the next Tab
-    // lands on the page's first control.
+    // lands on the page's first control. A click on the page's background focuses it too, which is
+    // the focus RESTING, not a choice a Back restore must respect (`FOCUS_REST_ATTRS`).
     <main
       ref={setRef}
       id={CONTENT_ID}
       tabIndex={-1}
+      {...FOCUS_REST_ATTRS}
       style={fade.style}
       className="flex-1 min-w-0 overflow-y-auto scrollbar-gutter-stable focus:outline-none"
     >