Feed: only the rows near the viewport are in the DOM, however far you scroll.

A long scroll left hundreds of rows mounted, and every repaint (a zap
count, a profile landing, the thirty-second sync) rebuilt and diffed all
of them — the slowdown deep in the feed. Rows beyond two and a half
screens either side of the viewport are now replaced by a spacer at their
measured height, so the page keeps its length (scrollbar, Back, arrival
anchoring behave as before) while a repaint touches a few dozen rows.
After svelte-virtual-list: measure after paint, estimate unmeasured rows
at the running average, hold the reader's row still when a measurement
corrects an estimate. Applies to the profile notes list too.

Tests: feed-window-dom (120 posts, 27,800px page, 12–24 rows mounted,
anchor held, spacers both sides); feed-live looks at the top after
scrolling up; the moderation test's stub relay no longer lets real posts in.

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

1 files changed, 127 insertions, 0 deletions

diff --git a/tools/feed-window-dom-test.js b/tools/feed-window-dom-test.js
new file mode 100644
index 0000000..d7a2f8c
--- /dev/null
+++ b/tools/feed-window-dom-test.js
@@ -0,0 +1,127 @@
+// Windowing: however far you scroll, only the rows near the viewport are in
+// the DOM — a spacer stands in for the rest at their measured height, so the
+// page keeps its length, the row under your eyes never moves when the window
+// shifts, and scrolling back up brings the top posts back.
+// Run: bun tools/feed-window-dom-test.js
+import puppeteer from 'puppeteer-core';
+import { cacheKeyFor } from '../src/wallet.js';
+import { generateSecretKey, getPublicKey, finalizeEvent } from 'nostr-tools/pure';
+import { buildHtml } from '../build.js';
+const sleep = (ms) => new Promise((r) => setTimeout(r, ms));
+let ok = true;
+const check = (n, c, d = '') => { console.log(` ${c ? '✓' : '✗'} ${n}${d ? ' — ' + d : ''}`); if (!c) ok = false; };
+const SK = generateSecretKey();
+const AUTHOR = getPublicKey(SK);
+const now = Math.floor(Date.now() / 1000);
+// 120 cached posts of wildly different heights
+const seeded = Array.from({ length: 120 }, (_, i) => finalizeEvent({
+  kind: 1, created_at: now - 100 - i * 60, tags: [],
+  content: 'post number ' + (i + 1) + ' ' + (i % 3 === 0 ? 'short' : i % 3 === 1 ? 'a medium length post about nothing in particular, '.repeat(3) : 'a long one. '.repeat(30)),
+}, SK));
+
+const html = await buildHtml({ minify: true, pwa: false });
+const server = Bun.serve({ port: 5305, fetch: (req) => {
+  const path = new URL(req.url).pathname;
+  if (path.startsWith('/punks')) return new Response(Bun.file('dist' + path));
+  return new Response(html, { headers: { 'content-type': 'text/html' } });
+} });
+const browser = await puppeteer.launch({ executablePath: '/usr/bin/google-chrome', headless: 'new', args: ['--no-sandbox'] });
+const page = await browser.newPage();
+// the stub relay holds every post: a kind-1 request is answered from it
+// (honouring until/limit), everything else gets a bare EOSE
+await page.evaluateOnNewDocument((all) => {
+  const Real = window.WebSocket;
+  window.WebSocket = function (...a) {
+    const ws = new Real(...a);
+    const send = ws.send.bind(ws);
+    ws.send = (data) => {
+      try {
+        const m = JSON.parse(data);
+        if (m[0] === 'REQ') {
+          const f = m[2] || {};
+          const evs = (f.kinds || []).includes(1) && !f['#e'] && !f.ids
+            ? all.filter((e) => (!f.until || e.created_at <= f.until) && (!f.since || e.created_at >= f.since)).slice(0, f.limit || 100) : [];
+          setTimeout(() => {
+            for (const e of evs) ws.dispatchEvent(new MessageEvent('message', { data: JSON.stringify(['EVENT', m[1], e]) }));
+            ws.dispatchEvent(new MessageEvent('message', { data: JSON.stringify(['EOSE', m[1]]) }));
+          }, 0);
+          return;
+        }
+        if (m[0] === 'EVENT') return;
+      } catch {}
+      return send(data);
+    };
+    return ws;
+  };
+  window.WebSocket.prototype = Real.prototype; Object.assign(window.WebSocket, Real);
+}, seeded);
+const errs = [];
+page.on('pageerror', (e) => errs.push(String(e)));
+const click = (t) => page.evaluate((x) => { const e = [...document.querySelectorAll('button')].find((n) => n.textContent.trim().toLowerCase().includes(x)); if (e) { e.click(); return true; } return false; }, t);
+const waitText = async (x, ms = 25000) => { for (let i = 0; i < ms / 250; i++) { if ((await page.evaluate(() => document.body.innerText)).toLowerCase().includes(x)) return true; await sleep(250); } return false; };
+const dom = () => page.evaluate(() => ({
+  rows: document.querySelectorAll('.notes-feed > .row[data-key]').length,
+  top: document.querySelector('.notes-feed > [data-key="win-top"]')?.offsetHeight || 0,
+  bottom: document.querySelector('.notes-feed > [data-key="win-bottom"]')?.offsetHeight || 0,
+  height: document.documentElement.scrollHeight, y: window.scrollY,
+  first: document.querySelector('.notes-feed > .row[data-key]')?.innerText.match(/post number \d+/)?.[0],
+}));
+const anchor = () => page.evaluate(() => {
+  const r = [...document.querySelectorAll('.notes-feed > .row[data-key]')].find((n) => { const b = n.getBoundingClientRect(); return b.bottom > 0 && b.top < innerHeight; });
+  return r ? { id: r.dataset.key, top: Math.round(r.getBoundingClientRect().top) } : null;
+});
+
+try {
+  await page.setViewport({ width: 390, height: 844 });
+  await page.goto('http://localhost:5305/', { waitUntil: 'domcontentloaded' });
+  await page.evaluate(() => localStorage.setItem('btc-wallet-network', 'regtest'));
+  await page.reload({ waitUntil: 'domcontentloaded' }); await sleep(400);
+  await click('create a new wallet'); await sleep(600);
+  await page.waitForSelector('.words .w .t');
+  const mn = (await page.$$eval('.words .w .t', (l) => l.map((e) => e.textContent.trim()))).join(' ');
+  await click('skip verification');
+  await waitText('receive', 20000);
+  const base = cacheKeyFor(mn + '\n');
+  await page.evaluate(([k, pk, ns]) => {
+    localStorage.setItem(k + ':follows', JSON.stringify({ tags: [['p', pk]], c: '', at: Math.floor(Date.now() / 1000) }));
+    localStorage.setItem(k + ':feedNotes', JSON.stringify(ns));
+    localStorage.setItem(k + ':profiles', JSON.stringify({ [pk]: { name: 'Prolific Poster', t: Date.now() } }));
+  }, [base, AUTHOR, seeded]);
+  await page.reload({ waitUntil: 'domcontentloaded' });
+  await waitText('receive', 20000);
+  await page.evaluate(() => { const b = [...document.querySelectorAll('button')].find((e) => /message/i.test(e.getAttribute('aria-label') || '')); if (b) b.click(); }); await sleep(800);
+  await page.evaluate(() => { const e = [...document.querySelectorAll('.item')].find((n) => (n.querySelector('.chat-name') || {}).textContent === 'Feed'); if (e) e.click(); });
+  await sleep(2500);
+  console.log('\n[a long scroll]');
+  check('the feed opens on its first posts', (await dom()).first === 'post number 1', JSON.stringify(await dom()));
+  // page to the bottom, again and again
+  let d = null;
+  for (let i = 0; i < 14; i++) {
+    await page.evaluate(() => window.scrollTo(0, document.documentElement.scrollHeight));
+    await sleep(700);
+    d = await dom();
+  }
+  check('many pages in, the page is long', d.height > 6000, d.height + 'px');
+  check('...but only a window of rows is mounted', d.rows > 0 && d.rows <= 60, d.rows + ' rows');
+  check('...with a spacer standing in for the rows above', d.top > 1000, d.top + 'px');
+
+  console.log('\n[the row under your eyes stays put]');
+  await page.evaluate(() => window.scrollTo(0, Math.round(document.documentElement.scrollHeight * 0.5))); await sleep(600);
+  const a1 = await anchor();
+  // the window moves as you read; a beat later, nothing under the eyes has shifted
+  await page.evaluate(() => window.scrollBy(0, 300)); await sleep(400);
+  await page.evaluate(() => window.scrollBy(0, -300)); await sleep(600);
+  const a2 = await anchor();
+  check('scrolling around and back leaves the same row at the same place', !!a1 && !!a2 && a1.id === a2.id && Math.abs(a1.top - a2.top) <= 2, JSON.stringify({ a1, a2 }));
+  const mid = await dom();
+  check('mid-feed, spacers stand in above and below', mid.top > 500 && mid.bottom > 500 && mid.rows <= 60, JSON.stringify(mid));
+
+  console.log('\n[back to the top]');
+  await page.evaluate(() => window.scrollTo(0, 0)); await sleep(800);
+  const t = await dom();
+  check('the first posts are back in the DOM', t.first === 'post number 1' && t.top === 0, JSON.stringify(t));
+  check('...and the rows below are held by a spacer', t.bottom > 1000 && t.rows <= 60, JSON.stringify(t));
+  check('no page errors', errs.length === 0, errs.slice(0, 2).join(' | '));
+} finally { await browser.close(); server.stop(true); }
+console.log(ok ? '\n✅ the feed stays light however far you scroll' : '\n❌ failed');
+process.exit(ok ? 0 : 1);