fix(threads): viewing a room no longer reads threads you don't follow (#314)

A passive read (just looking at the room) sent a threaded receipt for
every unread thread you don't follow, so the Threads panel's Unread
filter and Mark all read only ever worked for followed threads.
Decision (2026-10-19): leave them unread until opened, but stop
counting unfollowed threads toward the room's unread dot
(getUnreadInfo subtracts them like muted threads; a mention makes a
thread followed, so highlights are never hidden). isThreadFollowed
moves to utils/room. Adds a Playwright regression.

Fixes #314

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/notifications.ts

@@ -1,27 +1,19 @@
-import { MatrixClient, NotificationCountType, ReceiptType, Room, Thread } from 'matrix-js-sdk';
+import { MatrixClient, NotificationCountType, ReceiptType, Thread } from 'matrix-js-sdk';
 import { getSettings } from '../state/settings';
 import { readMarkedUnread, setMarkedUnread } from '../state/room/markedUnread';
+import { isThreadFollowed } from './room';
 
-/**
- * [Gitea #217] Threads the user is following: they started it, replied in it,
- * or were mentioned in it. `hasCurrentUserParticipated` comes from the server's
- * thread bundle and lags a reply we just sent, so our own loaded events count too.
- */
-export function isThreadFollowed(room: Room, thread: Thread, userId: string | null): boolean {
-  if (thread.hasCurrentUserParticipated) return true;
-  if (userId && thread.rootEvent?.getSender() === userId) return true;
-  if (userId && thread.timeline?.some((e) => e.getSender() === userId)) return true;
-  return (
-    (room.getThreadUnreadNotificationCount(thread.id, NotificationCountType.Highlight) ?? 0) > 0
-  );
-}
+// isThreadFollowed lives in utils/room (getUnreadInfo needs it too).
+export { isThreadFollowed };
 
 export type MarkAsReadOptions = {
   /**
    * [Gitea #217] The read comes from simply looking at the room (timeline at
-   * the bottom and focused), not an explicit "mark as read". Threads the user
-   * follows stay unread until their panel is opened, like Slack and Discord;
-   * the rest are cleared so they don't keep the room dot lit forever.
+   * the bottom and focused), not an explicit "mark as read". Threads stay
+   * unread until opened (or "Mark all read"), like Slack and Discord.
+   * [Gitea #314] That includes threads you don't follow; those no longer
+   * count toward the room's unread dot (see getUnreadInfo), so leaving them
+   * unread doesn't keep the room lit.
    */
   passive?: boolean;
   /**
@@ -66,7 +58,9 @@ export async function markAsRead(
   const threads = room.getThreads();
   const threadUnread = (thread: Thread) =>
     room.getThreadUnreadNotificationCount(thread.id, NotificationCountType.Total) ?? 0;
-  const keepThread = (thread: Thread) => passive && isThreadFollowed(room, thread, mx.getUserId());
+  // [Gitea #314] A passive read leaves every thread's state alone (it used to
+  // silently read every thread you don't follow).
+  const keepThread = (_thread: Thread) => passive;
   // [Gitea #217] An unthreaded receipt also reads every thread reply older than
   // it, so a passive read while a followed thread is unread must stay scoped
   // to the main timeline or the next main message would clear that thread.