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, 3 insertions, 0 deletions

diff --git a/src/style.css b/src/style.css
index 06ba80c..b136432 100644
--- a/src/style.css
+++ b/src/style.css
@@ -809,6 +809,9 @@ html.no-scroll { overflow: hidden; scrollbar-gutter: auto; }
 /* the profile notes feed: page-scrolled (no inner scrollbox); on phones it
    runs edge to edge — the card walls step outside #app's padding */
 .notes-feed { padding: 2px 14px; }
+/* rows scrolled well out of view are not in the DOM: a spacer stands in
+   at their measured height, so the page keeps its length (see listWindow) */
+.feed-spacer { display: block; flex: 0 0 auto; }
 @media (max-width: 520px) {
   .notes-feed {
     margin-left: -16px; margin-right: -16px;