Add backward prefetch for resumed mid-chapter reads

When the user resumes at a mid-chapter page, the reader previously
skipped the earlier pages entirely. This adds a scroll-up prefetch so
those earlier pages appear smoothly as the user scrolls toward the top.

- prependBatch() mirrors fetchBatch but decrements the offset cursor
  and prepends the new pages. prependExhaustedRef fires when the
  cursor hits 0 (start of chapter).
- Trigger: scrollY < 2500px fires prepend — well before the user
  reaches the top, so the DOM + images spawn ahead of the scroll
  position.
- Scroll preservation: aspect-[3/4] on <img> reserves vertical space
  before the image bytes arrive, so scrollHeight is accurate
  immediately after React commits. A single scrollBy(delta) keeps the
  previously-visible page visually anchored — no per-image jitter.
- Forward-fetch trigger indices + loadedCountRef are shifted by
  batch.length on each prepend so next-batch prefetch still fires at
  the correct page.

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

components/PageReader.tsx

@@ -1,9 +1,12 @@
 "use client";
 
-import { useState, useEffect, useRef, useCallback } from "react";
+import { useState, useEffect, useRef, useCallback, useMemo } from "react";
 import Link from "next/link";
 import { useRouter } from "next/navigation";
-import { writeLastReadChapter } from "@/components/ReadingProgressButton";
+import {
+  readProgress,
+  writeProgress,
+} from "@/components/ReadingProgressButton";
 
 type ChapterMeta = {
   id: number;
@@ -48,12 +51,30 @@ export function PageReader({
   const [pages, setPages] = useState<LoadedPage[]>([]);
   const [currentChapterNum, setCurrentChapterNum] =
     useState(startChapterNumber);
+  const [currentPageNum, setCurrentPageNum] = useState(() => {
+    if (typeof window === "undefined") return 1;
+    const p = readProgress(mangaSlug);
+    if (p && p.chapter === startChapterNumber && p.page > 1) return p.page;
+    return 1;
+  });
 
   const hiddenByScrollRef = useRef(false);
   const fetchChapterIdxRef = useRef(
     chapters.findIndex((c) => c.number === startChapterNumber)
   );
+  // Initialize offset from saved progress so the first fetch starts AT the
+  // user's last-read page — previous pages are skipped entirely
   const offsetRef = useRef(0);
+  const initialPageRef = useRef(1);
+  const offsetInitedRef = useRef(false);
+  if (!offsetInitedRef.current && typeof window !== "undefined") {
+    offsetInitedRef.current = true;
+    const p = readProgress(mangaSlug);
+    if (p && p.chapter === startChapterNumber && p.page > 1) {
+      offsetRef.current = p.page - 1;
+      initialPageRef.current = p.page;
+    }
+  }
   const loadingRef = useRef(false);
   const doneRef = useRef(false);
   // Count of pages already loaded — tracked via ref so fetchBatch stays stable
@@ -63,6 +84,13 @@ export function PageReader({
   const observerRef = useRef<IntersectionObserver | null>(null);
   const pageRefsRef = useRef<Map<number, HTMLDivElement>>(new Map());
 
+  // Reverse-fetch cursor: the offset of the first loaded page of the
+  // starting chapter. Used to prepend previous pages when the user scrolls
+  // up from a resumed mid-chapter position.
+  const prependOffsetRef = useRef(offsetRef.current);
+  const prependLoadingRef = useRef(false);
+  const prependExhaustedRef = useRef(offsetRef.current === 0);
+
   const advanceChapterOrFinish = useCallback(() => {
     if (fetchChapterIdxRef.current + 1 < chapters.length) {
       fetchChapterIdxRef.current += 1;
@@ -72,6 +100,71 @@ export function PageReader({
     }
   }, [chapters.length]);
 
+  const startChapter = useMemo(
+    () => chapters.find((c) => c.number === startChapterNumber),
+    [chapters, startChapterNumber]
+  );
+
+  const prependBatch = useCallback(async () => {
+    if (
+      prependLoadingRef.current ||
+      prependExhaustedRef.current ||
+      !startChapter
+    )
+      return;
+    const currentOffset = prependOffsetRef.current;
+    const newOffset = Math.max(0, currentOffset - BATCH_SIZE);
+    const limit = currentOffset - newOffset;
+    if (limit <= 0) {
+      prependExhaustedRef.current = true;
+      return;
+    }
+    prependLoadingRef.current = true;
+    const beforeHeight = document.documentElement.scrollHeight;
+    try {
+      const res = await fetch(
+        `/api/pages?chapterId=${startChapter.id}&offset=${newOffset}&limit=${limit}`
+      );
+      const batch: RawPage[] = await res.json();
+      if (batch.length === 0) {
+        prependExhaustedRef.current = true;
+        return;
+      }
+      prependOffsetRef.current = newOffset;
+      if (newOffset === 0) prependExhaustedRef.current = true;
+
+      // Shift forward-fetch trigger indices + total count since every
+      // already-loaded page has moved right by batch.length
+      const shift = batch.length;
+      const shifted = new Set<number>();
+      for (const t of triggerIndicesRef.current) shifted.add(t + shift);
+      triggerIndicesRef.current = shifted;
+      loadedCountRef.current += shift;
+
+      setPages((prev) => [
+        ...batch.map((p) => ({
+          chapterNumber: startChapter.number,
+          pageNumber: p.number,
+          imageUrl: p.imageUrl,
+        })),
+        ...prev,
+      ]);
+
+      // aspect-[3/4] on the prepended <img>s reserves height before their
+      // bytes arrive, so scrollHeight is accurate immediately after React
+      // commits. One scrollBy keeps the previously-visible page anchored.
+      requestAnimationFrame(() => {
+        const afterHeight = document.documentElement.scrollHeight;
+        const delta = afterHeight - beforeHeight;
+        if (delta > 0) window.scrollBy({ top: delta });
+      });
+    } catch {
+      // ignore — user can scroll-up again to retry
+    } finally {
+      prependLoadingRef.current = false;
+    }
+  }, [startChapter]);
+
   const fetchBatch = useCallback(async () => {
     if (loadingRef.current || doneRef.current) return;
     const chapter = chapters[fetchChapterIdxRef.current];
@@ -232,14 +325,29 @@ export function PageReader({
         hiddenByScrollRef.current = true;
         setShowUI(false);
       }
-      let current = startChapterNumber;
+      // Trigger backward prepend when user approaches the top — fire early
+      // so the DOM/pics are already spawned by the time the user scrolls there
+      if (
+        y < 2500 &&
+        !prependLoadingRef.current &&
+        !prependExhaustedRef.current
+      ) {
+        prependBatch();
+      }
+      // Nothing loaded yet — don't overwrite the resumed-page state
+      if (pages.length === 0) return;
+      let chapter = pages[0].chapterNumber;
+      let page = pages[0].pageNumber;
       for (let i = 0; i < pages.length; i++) {
         const el = pageRefsRef.current.get(i);
         if (!el) continue;
-        if (el.offsetTop <= y + 80) current = pages[i].chapterNumber;
-        else break;
+        if (el.offsetTop <= y + 80) {
+          chapter = pages[i].chapterNumber;
+          page = pages[i].pageNumber;
+        } else break;
       }
-      setCurrentChapterNum(current);
+      setCurrentChapterNum(chapter);
+      setCurrentPageNum(page);
     };
     const onScroll = () => {
       if (rafId) return;
@@ -250,12 +358,17 @@ export function PageReader({
       window.removeEventListener("scroll", onScroll);
       if (rafId) cancelAnimationFrame(rafId);
     };
-  }, [pages, startChapterNumber]);
+  }, [pages, startChapterNumber, prependBatch]);
 
-  // Persist reading progress whenever the visible chapter changes
+  // Persist progress as user scrolls. offsetRef has been pre-seeded above
+  // so the first fetched page IS the resumed page — no scroll restoration
+  // needed.
   useEffect(() => {
-    writeLastReadChapter(mangaSlug, currentChapterNum);
-  }, [mangaSlug, currentChapterNum]);
+    writeProgress(mangaSlug, {
+      chapter: currentChapterNum,
+      page: currentPageNum,
+    });
+  }, [mangaSlug, currentChapterNum, currentPageNum]);
 
   const currentChapter =
     chapters.find((c) => c.number === currentChapterNum) ??
@@ -345,7 +458,7 @@ export function PageReader({
               <img
                 src={page.imageUrl}
                 alt={`Page ${page.pageNumber}`}
-                className="w-full h-auto block align-bottom -mb-px [-webkit-touch-callout:none]"
+                className="w-full h-auto block align-bottom -mb-px aspect-[3/4] [-webkit-touch-callout:none]"
                 draggable={false}
               />
             </div>

components/ReadingProgressButton.tsx

@@ -13,34 +13,59 @@ type Props = {
   chapters: ChapterLite[];
 };
 
+export type ReadingProgress = {
+  chapter: number;
+  page: number;
+};
+
 function storageKey(slug: string) {
   return `sunnymh:last-read:${slug}`;
 }
 
-export function readLastReadChapter(slug: string): number | null {
+export function readProgress(slug: string): ReadingProgress | null {
   if (typeof window === "undefined") return null;
   const raw = window.localStorage.getItem(storageKey(slug));
   if (!raw) return null;
+  // New format: JSON { chapter, page }
+  if (raw.startsWith("{")) {
+    try {
+      const parsed = JSON.parse(raw) as ReadingProgress;
+      if (
+        typeof parsed.chapter === "number" &&
+        typeof parsed.page === "number" &&
+        parsed.chapter > 0 &&
+        parsed.page > 0
+      ) {
+        return parsed;
+      }
+    } catch {
+      return null;
+    }
+    return null;
+  }
+  // Legacy format: bare chapter number
   const n = Number(raw);
-  return Number.isFinite(n) && n > 0 ? n : null;
+  return Number.isFinite(n) && n > 0 ? { chapter: n, page: 1 } : null;
 }
 
-export function writeLastReadChapter(slug: string, chapter: number) {
+export function writeProgress(slug: string, progress: ReadingProgress) {
   if (typeof window === "undefined") return;
-  window.localStorage.setItem(storageKey(slug), String(chapter));
+  window.localStorage.setItem(storageKey(slug), JSON.stringify(progress));
 }
 
 export function ReadingProgressButton({ mangaSlug, chapters }: Props) {
-  const [lastRead, setLastRead] = useState<number | null>(null);
+  const [progress, setProgress] = useState<ReadingProgress | null>(null);
 
   useEffect(() => {
-    setLastRead(readLastReadChapter(mangaSlug));
+    setProgress(readProgress(mangaSlug));
   }, [mangaSlug]);
 
   if (chapters.length === 0) return null;
   const first = chapters[0];
   const resumeChapter =
-    lastRead !== null ? chapters.find((c) => c.number === lastRead) : null;
+    progress !== null
+      ? chapters.find((c) => c.number === progress.chapter)
+      : null;
   const target = resumeChapter ?? first;
 
   return (