A flick through the feed no longer drags background work through the middle of it. "When I flick to scroll, it's not getting up to speed very quickly, and clicking around in the app feels a bit sluggish at times" (Adam, 2026-11-07). Measured on his Pixel 9 in the installed PWA: idle is quiet (0.16 s of script in 10 s), but three real flicks through Following spent 1.7 s of 9 on the main thread, frames up to 200 ms, and the work was the avatar thumbnailer finishing faces as they scrolled in: a synchronous canvas.toDataURL WebP encode per face (229 ms), the profile cache JSON.stringified twice per half-second flush (once just to measure it, up to ~2 MB with thumbnails), and a whole-page repaint per finished face that changes nothing on screen (feed rows keep the face they were admitted with). Now: the crop, resize and encode happen off the main thread (createImageBitmap with resizeWidth/Height, OffscreenCanvas.convertToBlob, FileReader), with the old canvas path as the fallback; a finished face is recorded once the page settles; scheduleRepaint holds while a finger is down or the page scrolled in the last 200 ms (at most 1.5 s), and the profile flush likewise (at most 5 s, forced on an account switch) and sizes the blob by summing rows instead of stringifying it. Tests: the create flow's heads-up / passphrase / check steps from earlier today broke every test that made a wallet by waiting for the words and clicking "Skip verification"; tools/puppeteer.js exports seedWords() and skipSeedCheck() and the 96 tests use them. room-faces and avatar-profile fail the same way on 8858591 (row.hover, scrollIntoView), before today's changes

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01VMan6Ts5Tktega8VbLg31q

1 files changed, 71 insertions, 18 deletions

diff --git a/src/features/messages.js b/src/features/messages.js
index 90bfaaf..6c4f5f8 100644
--- a/src/features/messages.js
+++ b/src/features/messages.js
@@ -760,14 +760,38 @@ export function messagesFeature(ctx) {
   let allUnsubs = [];
   let dmUnsubs = []; // the DM wrap subscriptions alone, so they can be rebuilt
 
+  // Is the page under the reader's finger, or still coasting from a flick?
+  // Background work (a repaint for a face that arrived, writing the profile
+  // cache) waits for it to settle: on Adam's Pixel a flick through the feed
+  // spent 1.7 s of 9 on the main thread doing exactly that, and taps queued
+  // behind it (2026-11-07, "flick to scroll … not getting up to speed …
+  // clicking around feels sluggish"). Never longer than `most` ms, though:
+  // a slow, steady read must still see what arrived.
+  let pageMovedAt = 0, fingerDown = false;
+  if (typeof window !== 'undefined') {
+    window.addEventListener('scroll', () => { pageMovedAt = Date.now(); }, { passive: true, capture: true });
+    window.addEventListener('touchstart', () => { fingerDown = true; }, { passive: true, capture: true });
+    for (const n of ['touchend', 'touchcancel']) window.addEventListener(n, (e) => { if (!e.touches || !e.touches.length) fingerDown = false; }, { passive: true, capture: true });
+  }
+  const pageMoving = () => fingerDown || Date.now() - pageMovedAt < 200;
+  const whenSettled = (fn, most = 1500, step = 150) => {
+    const until = Date.now() + most;
+    const go = () => { if (pageMoving() && Date.now() < until) { setTimeout(go, step); return; } fn(); };
+    go();
+  };
+
   let repaintTimer = null;
   const scheduleRepaint = () => {
     if (repaintTimer) return;
+    const since = Date.now();
     const run = () => {
       // not under a pager in motion: the page beneath is out of sight, and
       // its repaint (40–55 ms on Adam's phone) dropped frames from flights
       // through the pictures (2026-10-17) — it waits for the landing
       if (pagerMoving()) { repaintTimer = setTimeout(run, 120); return; }
+      // nor mid-flick on the page itself (up to 1.5 s): a whole-page morph
+      // in the middle of a fling is a dropped frame and a late tap
+      if (pageMoving() && Date.now() - since < 1500) { repaintTimer = setTimeout(run, 150); return; }
       repaintTimer = null;
       // a visitor's public page (a profile, a thread, a feed) repaints too
       if (ui.screen === 'wallet' || ui.pubProf) renderThreadStable();
@@ -826,11 +850,14 @@ export function messagesFeature(ctx) {
   // memory and is written back once per burst: a batch of thirty profiles
   // landing during a scroll used to parse and re-serialize the whole blob
   // thirty times over, on the main thread, mid-scroll.
-  let profBlob = null, profFlush = null;
+  let profBlob = null, profFlush = null, profFlushSince = 0;
   const PROFILE_ROWS = 1500; // a row without a thumbnail is ~430 chars; IndexedDB has room
   const profilesDisk = () => (profBlob ||= wallet.loadFeatureState('profiles', {}));
-  function flushProfiles() {
-    profFlush = null;
+  function flushProfiles(force) {
+    // up to 5 s for the page to settle: stringifying this blob (thumbnails
+    // and all, up to ~2 MB) is a long main-thread task
+    if (!force && pageMoving() && Date.now() - (profFlushSince || 0) < 5000) { profFlush = setTimeout(flushProfiles, 300); return; }
+    profFlush = null; profFlushSince = 0;
     const s = profBlob;
     if (!s) return;
     // Who stays when space runs out: your own faces, the people you follow,
@@ -848,7 +875,10 @@ export function messagesFeature(ctx) {
     // Thumbnails are the bulk of this blob, so they live on a budget: the
     // least recently seen strangers give theirs up first. The row itself
     // stays — that face just paints the way it used to.
-    let size = JSON.stringify(s).length;
+    // (an estimate, not a JSON.stringify of the lot just to measure it:
+    // a row is ~430 chars plus its thumbnail)
+    let size = 0;
+    for (const k in s) size += 430 + ((s[k] && s[k].thumb) || '').length;
     if (size > THUMB_BUDGET) {
       // never our own: the header face is the one that must paint offline
       for (const k of evictFirst(Object.keys(s).filter((k) => s[k].thumb && !mine.has(k)))) {
@@ -875,7 +905,7 @@ export function messagesFeature(ctx) {
         ? { thumb: p.thumb, thumbFor: p.thumbFor, thumbPx: p.thumbPx || 0 } : {}),
       ...(p.thumbFail ? { thumbFail: p.thumbFail, thumbFailAt: p.thumbFailAt || 0, thumbFails: p.thumbFails || 1,
         thumbFailVersion: p.thumbFailVersion || 0 } : {}) };
-    if (!profFlush) profFlush = setTimeout(flushProfiles, 500);
+    if (!profFlush) { profFlushSince = Date.now(); profFlush = setTimeout(flushProfiles, 500); }
   }
   // A punk picture is OUR OWN art. Every coinos user who keeps the default
   // publishes https://v3.coinos.io/punks/N.webp as their nostr picture, and
@@ -1081,6 +1111,39 @@ export function messagesFeature(ctx) {
     blobProbing.set(url, task);
     return task;
   }
+  // Centre-crop to a square (the way background-size:cover paints it),
+  // shrink, and encode — off the main thread where the browser allows: the
+  // crop and resize through createImageBitmap, the encode through an
+  // OffscreenCanvas's convertToBlob. The synchronous canvas.toDataURL it
+  // replaces cost ~30 ms a face on a phone, mid-scroll (2026-11-07).
+  async function encodeThumb(bmp) {
+    const side = Math.min(bmp.width, bmp.height);
+    if (!side) throw new Error('empty');
+    const sx = (bmp.width - side) / 2, sy = (bmp.height - side) / 2;
+    if (typeof OffscreenCanvas !== 'undefined' && OffscreenCanvas.prototype.convertToBlob) {
+      let small = null;
+      try {
+        small = await createImageBitmap(bmp, sx, sy, side, side, { resizeWidth: THUMB_PX, resizeHeight: THUMB_PX, resizeQuality: 'high' });
+        const oc = new OffscreenCanvas(THUMB_PX, THUMB_PX);
+        oc.getContext('2d').drawImage(small, 0, 0);
+        const asUrl = (b) => new Promise((ok, bad) => { const r = new FileReader(); r.onload = () => ok(r.result); r.onerror = () => bad(r.error); r.readAsDataURL(b); });
+        let blob = await oc.convertToBlob({ type: 'image/webp', quality: 0.75 });
+        if (blob.type !== 'image/webp') blob = await oc.convertToBlob({ type: 'image/jpeg', quality: 0.7 });
+        if (blob.size * 4 / 3 > THUMB_MAX) blob = await oc.convertToBlob({ type: 'image/webp', quality: 0.5 }); // a busy picture, leaned on harder
+        const data = await asUrl(blob);
+        if (data.length > THUMB_MAX) throw new Error('too big');
+        return data;
+      } finally { try { small && small.close(); } catch {} }
+    }
+    const c = document.createElement('canvas');
+    c.width = c.height = THUMB_PX;
+    c.getContext('2d').drawImage(bmp, sx, sy, side, side, 0, 0, THUMB_PX, THUMB_PX);
+    let data = c.toDataURL('image/webp', 0.75);
+    if (!data.startsWith('data:image/webp')) data = c.toDataURL('image/jpeg', 0.7);
+    if (data.length > THUMB_MAX) data = c.toDataURL('image/webp', 0.5);
+    if (data.length > THUMB_MAX) throw new Error('too big');
+    return data;
+  }
   function makeThumb(pk, p) {
     if (!p || !p.picture || typeof document === 'undefined') return;
     if (noCorsHost(p.picture)) return; // a host that won't hand over bytes; the <img> shows it anyway
@@ -1152,18 +1215,8 @@ export function messagesFeature(ctx) {
           scheduleRepaint(); // the big face, where one is up, can paint from the copy now
         }
         bmp = await createImageBitmap(await res.blob());
-        // centre-crop to a square, the way background-size:cover paints it
-        const side = Math.min(bmp.width, bmp.height);
-        if (!side) throw new Error('empty');
-        const c = document.createElement('canvas');
-        c.width = c.height = THUMB_PX;
-        c.getContext('2d').drawImage(bmp, (bmp.width - side) / 2, (bmp.height - side) / 2,
-          side, side, 0, 0, THUMB_PX, THUMB_PX);
-        let data = c.toDataURL('image/webp', 0.75);
-        if (!data.startsWith('data:image/webp')) data = c.toDataURL('image/jpeg', 0.7);
-        if (data.length > THUMB_MAX) data = c.toDataURL('image/webp', 0.5); // a busy picture, leaned on harder
-        if (data.length > THUMB_MAX) throw new Error('too big');
-        done({ thumb: data, thumbFor: url, thumbPx: THUMB_PX });
+        const data = await encodeThumb(bmp);
+        whenSettled(() => done({ thumb: data, thumbFor: url, thumbPx: THUMB_PX }));
       } catch (e) {
         // a quick TypeError is the CORS refusal (a dead host takes longer)
         // only a refusal corsFetch confirmed (a body cut off mid-download
@@ -19062,7 +19115,7 @@ export function messagesFeature(ctx) {
       follows = null; followsAt = 0; feed = null; relayLists = null;
       mutes = null; mutesAt = 0;
       stCache = null; stAt = 0; // the next account's state, not this one's
-      if (profFlush) { clearTimeout(profFlush); flushProfiles(); } // this account's faces, written before the blob is dropped
+      if (profFlush) { clearTimeout(profFlush); flushProfiles(true); } // this account's faces, written before the blob is dropped
       profBlob = null;
       timedActivity.clear(); diskHeld.clear(); reacts.clear(); boosts.clear(); seenNoteEv.clear(); myReactEv.clear(); quoted.clear();
       noteCountsReady.clear();