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

Components/Content.razor

@@ -1,4 +1,5 @@
 @inherits LocalizableComponentBase
+@implements IHandleEvent
 
 @* One feed item: a post (a boost shows the original and its booster), a notification, or a conversation's last post.
    The card does its own Mastodon actions (favourite, boost, bookmark, reply, mute, block, delete) and tells the page
@@ -404,6 +405,45 @@
 
 	protected override void OnInitialized() => IsLoading = false;
 
+	// A card re-renders when something it shows may have changed: its own actions and toggles (its events), a new copy of
+	// its post (an edit, a grouped notification, a place found), where it is shown, or the persona and connection. Without
+	// this every card re-rendered whenever its list did: a feed of a thousand cards took seven minutes to scroll to
+	// (tests/e2e/Perf/FeedPerfTests).
+	object renderedWith;
+	int handlingEvents;
+
+	object RenderKey() => (Post, Post?.Status, Post?.Notification, Post?.Conversation, Post?.Location, Post?.GroupCount, Post?.GroupAccounts,
+		(FilterIn, Highlighted, Compact, IncludeExpand, CssContainer), (CascadingState?.PersonaId, CascadingState?.IsOnline));
+
+	protected override bool ShouldRender()
+	{
+		var key = RenderKey();
+		if (handlingEvents == 0 && Equals(key, renderedWith))
+			return false;
+		renderedWith = key;
+		return true;
+	}
+
+	// as ComponentBase handles an event, rendering after it and again when its task ends, with the card's own renders allowed
+	async Task IHandleEvent.HandleEventAsync(EventCallbackWorkItem callback, object arg)
+	{
+		handlingEvents++;
+		try
+		{
+			var task = callback.InvokeAsync(arg);
+			var pending = task.Status is not (TaskStatus.RanToCompletion or TaskStatus.Canceled);
+			StateHasChanged();
+			if (!pending)
+				return;
+			await task;
+			StateHasChanged();
+		}
+		finally
+		{
+			handlingEvents--;
+		}
+	}
+
 	RenderFragment Body(Status status) => @<div class="flex flex-col gap-3">
 		@if (status.Content is { Length: > 0 } content)
 		{