Feed images: the column copy and the grid tile are decoded, resized and WebP-encoded in a worker, one picture at a time, instead of on the page thread between scroll frames (a profile of the live feed showed 1.8 s of convertToBlob there, in tasks up to 383 ms). A picture that hangs or crashes the worker is given up and the worker restarts for the next; three in a row and originals paint for the session, as they do wherever workers can't run

Co-Authored-By: Claude Fable 5.1 <[email protected]>

2 files changed, 105 insertions, 37 deletions

diff --git a/src/features/messages.js b/src/features/messages.js
index 5cdf09b..e9e238a 100644
--- a/src/features/messages.js
+++ b/src/features/messages.js
@@ -33,6 +33,7 @@ import { loadMediaIndex, saveMediaIndex, wipeMediaIndex } from '../media-cache.j
 import { mergeFeedWindow } from '../feed-window.js';
 import { createThreadStore } from '../thread-cache.js';
 import { createThreadPrefetch } from '../thread-prefetch.js';
+import { makeImageThumbnails } from '../image-thumbnails.js';
 import { createFeedCache, FEED_CACHE_POSTS, FEED_SEED_POSTS } from '../feed-cache.js';
 import { POPULAR_RELAYS, FIREHOSE_RELAYS, popularCandidates } from '../popular-feed.js';
 import { makeSearcher, resultRows, fallbackAvatar, warmSearch, punkImageUrl, punkSmallUrl, primalCache } from '../recipient-search.js';
@@ -5732,28 +5733,14 @@ export function messagesFeature(ctx) {
       if (e && e.corsRefused) noteNoCors(url); // refused, as corsFetch confirmed
       return null;
     }
-    let bmp;
-    try { bmp = await createImageBitmap(blob); } catch { return null; }
-    const dims = { width: bmp.width, height: bmp.height };
-    // the Images grid's square comes from the same decode: scrolling the
-    // feed fills the grid's cache, no second download
-    if (!/gif/i.test(blob.type) && !tileThumbs.has(url)) await tileFromBitmap(url, bmp).catch(() => {});
-    // small enough already, or animated (a GIF's copy would be one frame):
-    // remember the shape and paint the original
-    const keep = bmp.width <= MEDIA_THUMB_W * 1.25 || /gif/i.test(blob.type);
-    let out = null;
-    if (!keep) {
-      try {
-        const h = Math.max(1, Math.round(bmp.height * MEDIA_THUMB_W / bmp.width));
-        const c = new OffscreenCanvas(MEDIA_THUMB_W, h);
-        const g = c.getContext('2d');
-        g.imageSmoothingQuality = 'high';
-        g.drawImage(bmp, 0, 0, MEDIA_THUMB_W, h);
-        out = await c.convertToBlob({ type: 'image/webp', quality: 0.82 });
-        if (!/webp/.test(out.type)) out = await c.convertToBlob({ type: 'image/jpeg', quality: 0.82 });
-      } catch { out = null; }
-    }
-    try { bmp.close(); } catch {}
+    const result = await makeImageThumbnails(blob, {
+      width: MEDIA_THUMB_W, tileSize: tileThumbs.has(url) ? 0 : TILE_PX,
+    });
+    if (!result) return null;
+    const dims = { width: result.width, height: result.height }, out = result.media;
+    // Both sizes share one decode in the worker; only cache writes and object
+    // URLs come back to the page. The lightbox still uses the original.
+    if (result.tile) await saveTileThumb(url, result.tile).catch(() => {});
     try {
       const store = await thumbStore();
       await store.put(url, new Response(out || new Blob(), { headers: { 'x-dims': JSON.stringify(dims), 'x-original': out ? '0' : '1' } }));
@@ -10561,22 +10548,10 @@ export function messagesFeature(ctx) {
       blob = await res.blob();
     } catch { return; }
     if (/gif/i.test(blob.type)) return; // animated: the tile shows the original
-    let bmp;
-    try { bmp = await createImageBitmap(blob); } catch { return; }
-    try { await tileFromBitmap(url, bmp); } catch {} finally { try { bmp.close(); } catch {} }
+    const result = await makeImageThumbnails(blob, { tileSize: TILE_PX });
+    if (result?.tile) await saveTileThumb(url, result.tile);
   }
-  // A tile's square, cut from a decoded picture and kept (the caller closes it).
-  async function tileFromBitmap(url, bmp) {
-    if (typeof OffscreenCanvas === 'undefined' || typeof caches === 'undefined') return;
-    const side = Math.min(bmp.width, bmp.height);
-    const px = Math.min(TILE_PX, side);
-    const sq = await createImageBitmap(bmp, (bmp.width - side) / 2, (bmp.height - side) / 2, side, side,
-      { resizeWidth: px, resizeHeight: px, resizeQuality: 'high' });
-    const c = new OffscreenCanvas(px, px);
-    c.getContext('2d').drawImage(sq, 0, 0);
-    try { sq.close(); } catch {}
-    let out = await c.convertToBlob({ type: 'image/webp', quality: 0.8 });
-    if (!/webp/.test(out.type)) out = await c.convertToBlob({ type: 'image/jpeg', quality: 0.8 });
+  async function saveTileThumb(url, out) {
     tileThumbs.set(url, URL.createObjectURL(out));
     const store = await tileStore();
     await store.put(tileKey(url), new Response(out, { headers: { 'content-type': out.type } }));
diff --git a/src/image-thumbnails.js b/src/image-thumbnails.js
new file mode 100644
index 0000000..ffac4b9
--- /dev/null
+++ b/src/image-thumbnails.js
@@ -0,0 +1,93 @@
+// Keep decode, resize and WebP encoding off the scrolling thread. A blob
+// worker also works in the standalone wallet without a separately hosted file.
+// This function is serialized: all its dependencies must live inside it.
+function thumbnailWorker() {
+  const encode = async (canvas, quality) => {
+    let blob = await canvas.convertToBlob({ type: 'image/webp', quality });
+    if (!/webp/.test(blob.type)) blob = await canvas.convertToBlob({ type: 'image/jpeg', quality });
+    return blob;
+  };
+  self.onmessage = async ({ data: { blob, width, tileSize } }) => {
+    let bitmap;
+    try {
+      bitmap = await createImageBitmap(blob);
+      const result = { width: bitmap.width, height: bitmap.height, media: null, tile: null };
+      // GIFs must keep animating; a canvas copy would freeze their first frame.
+      if (!/gif/i.test(blob.type)) {
+        if (tileSize) {
+          const side = Math.min(bitmap.width, bitmap.height), px = Math.min(tileSize, side);
+          let square;
+          try {
+            square = await createImageBitmap(bitmap, (bitmap.width - side) / 2, (bitmap.height - side) / 2, side, side,
+              { resizeWidth: px, resizeHeight: px, resizeQuality: 'high' });
+            const canvas = new OffscreenCanvas(px, px);
+            canvas.getContext('2d').drawImage(square, 0, 0);
+            result.tile = await encode(canvas, 0.8);
+          } catch {} finally { square?.close(); }
+        }
+        if (width && bitmap.width > width * 1.25) {
+          try {
+            const height = Math.max(1, Math.round(bitmap.height * width / bitmap.width));
+            const canvas = new OffscreenCanvas(width, height), ctx = canvas.getContext('2d');
+            ctx.imageSmoothingQuality = 'high';
+            ctx.drawImage(bitmap, 0, 0, width, height);
+            result.media = await encode(canvas, 0.82);
+          } catch {}
+        }
+      }
+      self.postMessage(result);
+    } catch { self.postMessage(null); } finally { bitmap?.close(); }
+  };
+}
+
+// One image at a time bounds concurrent decoding/encoding and its CPU/memory
+// pressure. If workers are unavailable, callers keep displaying originals.
+const JOB_MS = 15000, MAX_QUEUE = 24, MAX_STRIKES = 3;
+let worker = null, disabled = false, current = null, timer = null, strikes = 0;
+const queue = [];
+function finish(job, data) { clearTimeout(timer); timer = null; job?.resolve(data); }
+// Workers can't be made here (constructor refused, script won't load):
+// thumbnails are off for this session.
+function failWorker() {
+  disabled = true;
+  worker?.terminate(); worker = null;
+  const job = current; current = null;
+  finish(job, null);
+  for (const j of queue.splice(0)) j.resolve(null);
+}
+// A picture that hung or crashed the worker is given up and the worker
+// starts fresh for the next one — one bad file mustn't cost every later
+// thumbnail. Three failures in a row and it's treated as broken.
+function resetWorker() {
+  if (++strikes >= MAX_STRIKES) return failWorker();
+  worker?.terminate(); worker = null;
+  const job = current; current = null;
+  finish(job, null);
+  drain();
+}
+function drain() {
+  if (current || !queue.length || disabled) return;
+  if (!worker) {
+    let url;
+    try {
+      url = URL.createObjectURL(new Blob(['(' + thumbnailWorker.toString() + ')()'], { type: 'text/javascript' }));
+      worker = new Worker(url, { name: 'image-thumbnails' });
+      worker.onerror = resetWorker;
+      worker.onmessageerror = resetWorker;
+      worker.onmessage = ({ data }) => {
+        strikes = 0;
+        const job = current; current = null;
+        finish(job, data);
+        drain();
+      };
+    } catch { failWorker(); return; } finally { if (url) URL.revokeObjectURL(url); }
+  }
+  current = queue.shift();
+  timer = setTimeout(resetWorker, JOB_MS);
+  try { worker.postMessage(current.input); } catch { resetWorker(); }
+}
+export function makeImageThumbnails(blob, { width = 0, tileSize = 0 } = {}) {
+  if (disabled || typeof Worker === 'undefined' || typeof OffscreenCanvas === 'undefined'
+    || typeof createImageBitmap === 'undefined' || queue.length >= MAX_QUEUE) return Promise.resolve(null);
+  return new Promise((resolve) => { queue.push({ input: { blob, width, tileSize }, resolve }); drain(); });
+}