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, 4 insertions, 1 deletions

diff --git a/tools/feed-live-test.js b/tools/feed-live-test.js
index 7732045..9f98fde 100644
--- a/tools/feed-live-test.js
+++ b/tools/feed-live-test.js
@@ -287,7 +287,10 @@ try {
   await away(() => Array.from({ length: 25 }, (_, i) => signed('LONG AWAY ' + (i + 1), 0)));
   const a3 = await rowAt(heldLong.text);
   check('many new posts still preserve the reading position', !!a3 && Math.abs(a3.top - heldLong.top) <= 2, JSON.stringify({ heldLong, a3 }));
-  check('...with the newest posts above and a notice', /LONG AWAY 1\b/.test(await topRow()) && !!(await page.$('.feed-new-pill')), (await topRow()).slice(0, 40));
+  check('...with a notice', !!(await page.$('.feed-new-pill')));
+  // the page top is not mounted from down here (windowing): go up to look
+  await page.evaluate(() => window.scrollTo(0, 0)); await sleep(600);
+  check('...and the newest posts at the top', /LONG AWAY 1\b/.test(await topRow()), (await topRow()).slice(0, 40));
 
   check('no page errors', errs.length === 0, errs.slice(0, 2).join(' | '));
 } finally { await browser.close(); server.stop(true); }