Preload the next page in page mode

CURRENT.md

@@ -45,7 +45,8 @@ mobile draft at 390px.
     number / total / Next, persists the page in localStorage, and scrolls to the next page one at
     a time with a 2 s cooldown after each load. Same filter + page is stable while the archive is
     unchanged. Infinite mode keeps the stream buffer; prefetch, buffer fill, and the buffer are
-    disabled in page mode.
+    disabled in page mode. Page mode pre-loads the next page's JSON and preview images, so
+    jumping one page renders instantly.
   - [x] C56 Recovery completed. First run after the C55 fix recovered 1,117 of 1,240 Discord-CDN
     previews (90%); the last 123 were confirmed genuinely gone (refresh + direct fetch and `/stream`
     both fail) and are marked `unavailable after recovery` so later runs skip them. The recovery

poc/public/media.js

@@ -36,6 +36,7 @@
         pageMode: localStorage.getItem("vk-media-page-mode") === "page" ? "page" : "infinite",
         page: Math.max(1, Number(localStorage.getItem("vk-media-page")) || 1),
         pageCooldownUntil: 0,
+        pagePrefetch: null,
         draft: null,
         prefetched: null,
         prefetchPromise: null,
@@ -414,9 +415,17 @@
         }
 
         try {
-            const items = await window.VK.api.get(
-                `/api/media?limit=${state.pageSize}&offset=${state.offset}&${filterQuery()}`
-            );
+            const prefetched =
+                reset && state.pageMode === "page" && state.pagePrefetch?.page === state.page
+                    ? state.pagePrefetch
+                    : null;
+            if (prefetched) state.pagePrefetch = null;
+
+            const items =
+                prefetched?.items ??
+                (await window.VK.api.get(
+                    `/api/media?limit=${state.pageSize}&offset=${state.offset}&${filterQuery()}`
+                ));
 
             if (reset) {
                 for (const item of items) state.itemKeys.add(`${item.scrapeId}:${item.itemId}`);
@@ -442,13 +451,44 @@
         } else {
             state.pageCooldownUntil = Date.now() + 2000;
             renderToolbar();
+            void prefetchNext();
         }
     }
 
     /** Warm the next page's JSON and preview images before the user reaches them. */
+    function warmImages(items) {
+        for (const item of items) {
+            const src = item.thumbUrl || item.localUrl;
+            if (!src) continue;
+            const image = new Image();
+            image.src = src;
+        }
+    }
+
+    /**
+     * Warm the next slice in both modes. Infinite appends it when the user reaches the bottom;
+     * page mode keeps it ready so jumping one page renders instantly with its previews cached.
+     */
     async function prefetchNext() {
-        if (state.pageMode !== "infinite") return;
-        if (!state.loaded || !state.more || state.prefetched || state.prefetchPromise || state.loading) return;
+        if (!state.loaded || state.loading) return;
+
+        if (state.pageMode === "page") {
+            const page = state.page + 1;
+            if (!hasNextPage() || state.pagePrefetch?.page === page) return;
+
+            try {
+                const items = await window.VK.api.get(
+                    `/api/media?limit=${state.pageSize}&offset=${(page - 1) * state.pageSize}&${filterQuery()}`
+                );
+                state.pagePrefetch = { page, items };
+                warmImages(items);
+            } catch {
+                // best effort
+            }
+            return;
+        }
+
+        if (!state.more || state.prefetched || state.prefetchPromise) return;
 
         state.prefetchPromise = window.VK.api
             .get(`/api/media?limit=${state.pageSize}&offset=${state.offset}&${filterQuery()}`)
@@ -459,12 +499,7 @@
                 }
 
                 state.prefetched = items;
-                for (const item of items) {
-                    const src = item.thumbUrl || item.localUrl;
-                    if (!src) continue;
-                    const image = new Image();
-                    image.src = src;
-                }
+                warmImages(items);
             })
             .catch(() => {
                 // Preloading is best effort; the scroll handler falls back to a request.
@@ -564,6 +599,7 @@
     function setPageMode(mode) {
         if (state.pageMode === mode) return;
         state.pageMode = mode;
+        state.pagePrefetch = null;
         localStorage.setItem("vk-media-page-mode", mode);
         state.prefetched = null;
         state.page = 1;
@@ -573,6 +609,7 @@
     }
 
     function gotoPage(page) {
+        state.pagePrefetch = null;
         const count = pageCount();
         const wanted = Math.max(1, Math.round(Number(page) || 1));
         state.page = count ? Math.min(count, wanted) : wanted;
@@ -1476,6 +1513,7 @@
 
     function applyDraft() {
         state.page = 1;
+        state.pagePrefetch = null;
         savePage();
         state.kind = state.draft.kind;
         state.filters = {
@@ -1996,6 +2034,7 @@
         state.more = false;
         state.page = 1;
         state.pageCooldownUntil = 0;
+        state.pagePrefetch = null;
         savePage();
         state.prefetched = null;
         state.items = [];
@@ -2042,6 +2081,7 @@
     overlay.querySelector("#media-close").addEventListener("click", hide);
     overlay.querySelector("#media-refresh").addEventListener("click", () => {
         state.completed.clear();
+        state.pagePrefetch = null;
         void loadScrapes();
         if (state.view === "media") void loadMedia(true);
         if (state.view === "bot") {