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/room.ts

@@ -18,6 +18,7 @@ import {
   NotificationCountType,
   RelationType,
   Room,
+  Thread,
   RoomMember,
   TimelineEvents,
   IRoomTimelineData,
@@ -418,6 +419,20 @@ export const roomHaveUnread = (mx: MatrixClient, room: Room) => {
   return true;
 };
 
+/**
+ * [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
+  );
+}
+
 export const getUnreadInfo = (
   room: Room,
   mutedThreads?: Set<string>,
@@ -439,6 +454,18 @@ export const getUnreadInfo = (
     if (highlight < 0) highlight = 0;
   }
 
+  // [Gitea #314] Threads you don't follow stay unread in the Threads panel
+  // (viewing the room no longer reads them) but don't light the room's dot.
+  // A mention makes a thread followed, so highlights are never subtracted.
+  if (mx) {
+    room.getThreads().forEach((thread) => {
+      if (mutedThreads?.has(thread.id)) return;
+      if (isThreadFollowed(room, thread, mx.getUserId())) return;
+      total -= room.getThreadUnreadNotificationCount(thread.id, NotificationCountType.Total) ?? 0;
+    });
+    if (total < 0) total = 0;
+  }
+
   const resolvedTotal = highlight > total ? highlight : total;
 
   // Suppress a spurious Total when the room isn't really unread. Two safe cases,