The feed stays light at thousands of posts

A Perf test (tests/e2e/Perf/FeedPerfTests, Category=Perf) scrolls load0's
home, thousands of posts from SocialPub's load runs, to 2000 posts. Before:
100 cards took 12 s to reach, 1000 took 437 s, 2000 took 29 minutes; the
main thread was busy almost all of that time, and the page held 224 000 DOM
nodes and 920 MB of .NET heap. Every card re-rendered whenever the list did.

- A card re-renders only when its own events run or its render key changes
  (Content.RenderKey: the post, its status, notification, conversation,
  place and grouping, where it is shown, the persona and the connection).
  2000 cards then took 122 s.
- The list is a <Virtualize>: only the cards near the view are rendered.
  The page now holds about 3 000 DOM nodes and 145 MB however long the feed
  is, and each page of older posts costs the same (about 1.9 s in the test's
  scroll loop) at the 100th post as at the 1700th. The feed tells how many
  posts it holds in data-feed-count.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Pages/Index.razor

@@ -23,7 +23,7 @@
 
 		<div class="relative flex-1 min-h-0 neo-inset neo-xs rounded-xl">
 			<div id="@ScrollId" class="absolute inset-0 overflow-y-auto p-3 md:p-4">
-				<div class="flex flex-col gap-4">
+				<div class="flex flex-col gap-4" data-feed-count="@posts.Count">
 
 					@if (isComposing)
 					{
@@ -74,10 +74,11 @@
 						</p>
 					}
 
-					@foreach (var post in posts)
-					{
+					@* only the cards near the view are rendered: a feed of thousands stays as light as one of forty
+					   (tests/e2e/Perf/FeedPerfTests) *@
+					<Virtualize Items="posts" Context="post" ItemSize="320" OverscanCount="8">
 						<Content @key="post.Key" Post="post" FilterIn="FilterIn" OnDeleted="Remove" OnReplied="Replied" OnTag="OpenTag" />
-					}
+					</Virtualize>
 
 					<div id="@SentinelId" class="flex justify-center min-h-8 text-xs opacity-60">
 						@if (isLoadingMore)