perf(feed): smoother scrolling for long feeds

Three low-risk wins for large filtered feeds:
- content-visibility:auto on cards (.cv-card/.cv-row) so the browser skips
  layout/paint for off-screen cards; contain-intrinsic-size keeps the scrollbar
  stable and is remembered per card after first render.
- memo(VideoCard) + stable onState/onChannelFilter callbacks (onState reads the
  loaded list via a ref) so appending a page only renders the ~60 new cards
  instead of reconciling every card already on screen.
- Prefetch the next page earlier (sentinel rootMargin 800px → 1500px) so the
  'Loading more…' flash is far less likely during fast scrolling.

frontend/src/components/Feed.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useRef, useState } from "react";
+import { useCallback, useEffect, useRef, useState } from "react";
 import { useInfiniteQuery, useQuery, useQueryClient } from "@tanstack/react-query";
 import { api, type FeedFilters, type Video } from "../lib/api";
 import { notify } from "../lib/notifications";
@@ -66,13 +66,19 @@ export default function Feed({
           fetchNextPage();
         }
       },
-      { rootMargin: "800px" }
+      { rootMargin: "1500px" } // prefetch the next page well before the bottom is in view
     );
     io.observe(el);
     return () => io.disconnect();
   }, [hasNextPage, isFetchingNextPage, fetchNextPage]);
 
-  function onState(id: string, status: string) {
+  // Keep the loaded videos in a ref so onState can stay referentially stable
+  // (it needs the list only to look up a title for the notification). A stable
+  // onState lets memo(VideoCard) skip re-rendering existing cards on page append.
+  const loadedRef = useRef<Video[]>([]);
+
+  const onState = useCallback(
+    (id: string, status: string) => {
       setOverrides((o) => ({ ...o, [id]: status }));
       // Refetch once the server has the change so other views (e.g. Hidden) are in sync.
       api
@@ -80,7 +86,7 @@ export default function Feed({
         .then(() => qc.invalidateQueries({ queryKey: ["feed"] }))
         .catch(() => {});
       if (status === "hidden") {
-      const v = (query.data?.pages ?? []).flatMap((p) => p.items).find((x) => x.id === id);
+        const v = loadedRef.current.find((x) => x.id === id);
         notify({
           message: v?.title ? `Hidden “${v.title}”` : "Video hidden",
           action: { label: "Undo", onClick: () => onState(id, "new") },
@@ -93,21 +99,27 @@ export default function Feed({
           },
         });
       } else if (status === "watched") {
-      const v = (query.data?.pages ?? []).flatMap((p) => p.items).find((x) => x.id === id);
+        const v = loadedRef.current.find((x) => x.id === id);
         notify({
           message: v?.title ? `Marked watched “${v.title}”` : "Marked watched",
           action: { label: "Unwatch", onClick: () => onState(id, "new") },
           meta: { kind: "video-watched", videoId: id, title: v?.title ?? "this video" },
         });
       }
-  }
+    },
+    [qc]
+  );
 
-  function onChannelFilter(channelId: string, channelName: string) {
+  const onChannelFilter = useCallback(
+    (channelId: string, channelName: string) => {
       setFilters({ ...filters, channelId, channelName });
-  }
+    },
+    [filters, setFilters]
+  );
 
-  const items: Video[] = (query.data?.pages ?? [])
-    .flatMap((p) => p.items)
+  const loaded: Video[] = (query.data?.pages ?? []).flatMap((p) => p.items);
+  loadedRef.current = loaded;
+  const items: Video[] = loaded
     .map((v) => (overrides[v.id] ? { ...v, status: overrides[v.id] } : v))
     .filter((v) => matchesView(v.status, filters.show));
 

frontend/src/components/VideoCard.tsx

@@ -1,3 +1,4 @@
+import { memo } from "react";
 import { Bookmark, Check, CheckCheck, Eye, EyeOff, ListFilter } from "lucide-react";
 import Avatar from "./Avatar";
 import clsx from "clsx";
@@ -136,7 +137,7 @@ function Thumb({
   );
 }
 
-export default function VideoCard({
+function VideoCard({
   video,
   view,
   onState,
@@ -172,7 +173,7 @@ export default function VideoCard({
     return (
       <div
         className={clsx(
-          "group flex gap-3 p-2 rounded-xl hover:bg-card hover:shadow-lg transition",
+          "cv-row group flex gap-3 p-2 rounded-xl hover:bg-card hover:shadow-lg transition",
           watched && "opacity-55"
         )}
       >
@@ -198,7 +199,7 @@ export default function VideoCard({
   return (
     <div
       className={clsx(
-        "group glass-card glass-hover rounded-2xl p-2.5 transition-all duration-150 hover:-translate-y-1",
+        "cv-card group glass-card glass-hover rounded-2xl p-2.5 transition-all duration-150 hover:-translate-y-1",
         watched && "opacity-55"
       )}
     >
@@ -227,3 +228,8 @@ export default function VideoCard({
     </div>
   );
 }
+
+// Memoized so appending a feed page only renders the new cards, not every existing
+// one (the callbacks from Feed are stable; non-overridden video objects keep their
+// identity across renders).
+export default memo(VideoCard);

frontend/src/index.css

@@ -72,6 +72,19 @@ html[data-perf="1"] body {
   background: var(--bg);
 }
 
+/* Skip layout/paint for off-screen feed cards so long lists stay smooth while
+   scrolling. contain-intrinsic-size reserves an approximate box so the scrollbar
+   stays stable; `auto` lets the browser remember each card's real size after it
+   has rendered once. */
+.cv-card {
+  content-visibility: auto;
+  contain-intrinsic-size: auto 320px;
+}
+.cv-row {
+  content-visibility: auto;
+  contain-intrinsic-size: auto 96px;
+}
+
 /* ===== Motion ===== */
 @keyframes fadeIn {
   from { opacity: 0; }