Center current chapter in drawer on open

Chapter drawer now opens with the active row pre-scrolled to the center
of the list instead of always starting at chapter #1. useLayoutEffect
measures via getBoundingClientRect so the scroll lands before paint —
no visible jump.

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

components/PageReader.tsx

@@ -86,6 +86,8 @@ export function PageReader({
   const pageElRef = useRef<Map<string, HTMLDivElement>>(new Map());
   const observerRef = useRef<IntersectionObserver | null>(null);
   const hiddenByScrollRef = useRef(false);
+  const drawerScrollRef = useRef<HTMLDivElement | null>(null);
+  const drawerActiveRef = useRef<HTMLAnchorElement | null>(null);
   // Pages currently inside the observer's viewport margin. The scroll tick
   // walks this small set instead of every loaded page.
   const intersectingPagesRef = useRef<Map<string, IntersectingPage>>(new Map());
@@ -322,6 +324,21 @@ export function PageReader({
     []
   );
 
+  useLayoutEffect(() => {
+    if (!showDrawer) return;
+    const scroll = drawerScrollRef.current;
+    const active = drawerActiveRef.current;
+    if (!scroll || !active) return;
+    const scrollRect = scroll.getBoundingClientRect();
+    const activeRect = active.getBoundingClientRect();
+    const delta =
+      activeRect.top -
+      scrollRect.top -
+      scroll.clientHeight / 2 +
+      active.clientHeight / 2;
+    scroll.scrollTop = Math.max(0, scroll.scrollTop + delta);
+  }, [showDrawer]);
+
   const currentChapter =
     chapters.find((c) => c.number === currentChapterNum) ??
     chapters.find((c) => c.number === startChapterNumber);
@@ -474,12 +491,16 @@ export function PageReader({
                 </span>
               </div>
             </div>
-            <div className="overflow-y-auto max-h-[calc(75vh-4rem)] pb-safe">
+            <div
+              ref={drawerScrollRef}
+              className="overflow-y-auto max-h-[calc(75vh-4rem)] pb-safe"
+            >
               {chapters.map((ch) => {
                 const isActive = ch.number === currentChapterNum;
                 return (
                   <Link
                     key={ch.number}
+                    ref={isActive ? drawerActiveRef : undefined}
                     href={`/manga/${mangaSlug}/${ch.number}`}
                     scroll={false}
                     className={`flex items-center gap-3 px-5 py-3 text-sm transition-colors ${