Sync URL and prev/next nav with the chapter being viewed

Once continuous scroll crosses a chapter boundary, the URL was stuck at
the originally-opened chapter so browser back / reload would jump the
user back there. Double-tap left/right also walked off the wrong chapter
since prevChapter/nextChapter were frozen at mount time.

- replaceState the URL as currentChapterNum changes (no server refetch).
- Derive prevChapter/nextChapter dynamically via useMemo on
  currentChapterNum, dropping the now-redundant server-computed props.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/manga/[slug]/[chapter]/page.tsx

@@ -50,16 +50,6 @@ export default async function ChapterReaderPage({ params }: Props) {
   const currentChapter = manga.chapters.find((c) => c.number === chapterNum);
   if (!currentChapter) notFound();
 
-  const chapterIndex = manga.chapters.findIndex(
-    (c) => c.number === chapterNum
-  );
-  const prevChapter =
-    chapterIndex > 0 ? manga.chapters[chapterIndex - 1].number : null;
-  const nextChapter =
-    chapterIndex < manga.chapters.length - 1
-      ? manga.chapters[chapterIndex + 1].number
-      : null;
-
   const allChapters = manga.chapters.map((c) => ({
     id: c.id,
     number: c.number,
@@ -72,8 +62,6 @@ export default async function ChapterReaderPage({ params }: Props) {
       mangaSlug={manga.slug}
       mangaTitle={manga.title}
       startChapterNumber={currentChapter.number}
-      prevChapter={prevChapter}
-      nextChapter={nextChapter}
       chapters={allChapters}
       initialChapterMeta={initialChapterMeta}
     />

components/PageReader.tsx

@@ -29,8 +29,6 @@ type PageReaderProps = {
   mangaSlug: string;
   mangaTitle: string;
   startChapterNumber: number;
-  prevChapter: number | null;
-  nextChapter: number | null;
   chapters: ChapterMeta[];
   initialChapterMeta: PageMeta[];
 };
@@ -51,8 +49,6 @@ export function PageReader({
   mangaSlug,
   mangaTitle,
   startChapterNumber,
-  prevChapter,
-  nextChapter,
   chapters,
   initialChapterMeta,
 }: PageReaderProps) {
@@ -272,6 +268,25 @@ export function PageReader({
     });
   }, [mangaSlug, currentChapterNum, currentPageNum]);
 
+  // Keep URL in sync with the chapter currently in the viewport so browser
+  // back / reload returns to the latest chapter, not the one first opened.
+  useEffect(() => {
+    const url = `/manga/${mangaSlug}/${currentChapterNum}`;
+    if (window.location.pathname === url) return;
+    window.history.replaceState(window.history.state, "", url);
+  }, [mangaSlug, currentChapterNum]);
+
+  const { prevChapter, nextChapter } = useMemo(() => {
+    const idx = chapters.findIndex((c) => c.number === currentChapterNum);
+    return {
+      prevChapter: idx > 0 ? chapters[idx - 1].number : null,
+      nextChapter:
+        idx >= 0 && idx < chapters.length - 1
+          ? chapters[idx + 1].number
+          : null,
+    };
+  }, [chapters, currentChapterNum]);
+
   const router = useRouter();
   const touchMovedRef = useRef(false);
   const singleTapTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);