feat(threads): Slack-style per-thread notifications (P4-1)

Default = Participating: thread replies notify only when you've posted in the
thread or are @mentioned; per-thread override to All / Mentions-only / Mute via
a bell menu in the thread panel header. Modes sync across devices in
io.lotus.thread_notifications account data (pruned on write: left rooms, >180d,
cap 200/room). Muted threads: no notifications/sounds, chip badge suppressed
(+BellMute glyph), and their counts are subtracted from the room's sidebar
badge (client-side; clamped ≥0).

Also fixes the thread notification path itself: thread replies are now owned by
exactly ONE handler (room-level ThreadEvent.NewReply via a new useRoomsListener
hook, with per-thread dedupe, panel-aware focus suppression, and per-thread OS
tag coalescing) — the existing RoomEvent.Timeline handlers in the notifier and
the unread binder are explicitly thread-guarded, eliminating the previously
un-gated/double path. Room badges now also refresh live on
RoomEvent.UnreadNotifications (surgical per-room PUT; fixes thread-badge lag).

Pure decision core shouldNotifyThreadReply (13-case matrix) + prune + unread
subtraction: +32 tests (648 total). E2EE caveat documented: mentions-only may
under-notify pre-decryption (same class as the existing path).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/state/hooks/useBindAtoms.ts

@@ -5,12 +5,14 @@ import { mDirectAtom, useBindMDirectAtom } from '../mDirectList';
 import { roomToUnreadAtom, useBindRoomToUnreadAtom } from '../room/roomToUnread';
 import { roomToParentsAtom, useBindRoomToParentsAtom } from '../room/roomToParents';
 import { roomIdToTypingMembersAtom, useBindRoomIdToTypingMembersAtom } from '../typingMembers';
+import { threadNotificationsAtom, useBindThreadNotificationsAtom } from '../threadNotifications';
 
 export const useBindAtoms = (mx: MatrixClient) => {
   useBindMDirectAtom(mx, mDirectAtom);
   useBindAllInvitesAtom(mx, allInvitesAtom);
   useBindAllRoomsAtom(mx, allRoomsAtom);
   useBindRoomToParentsAtom(mx, roomToParentsAtom);
+  useBindThreadNotificationsAtom(mx, threadNotificationsAtom);
   useBindRoomToUnreadAtom(mx, roomToUnreadAtom);
 
   useBindRoomIdToTypingMembersAtom(mx, roomIdToTypingMembersAtom);

src/app/state/room/roomToUnread.ts

@@ -1,15 +1,16 @@
 import { produce } from 'immer';
-import { atom, useSetAtom } from 'jotai';
+import { atom, useAtomValue, useSetAtom } from 'jotai';
 import {
   IRoomTimelineData,
   MatrixClient,
   MatrixEvent,
+  NotificationCount,
   Room,
   RoomEvent,
   SyncState,
 } from 'matrix-js-sdk';
 import { ReceiptContent, ReceiptType } from 'matrix-js-sdk/lib/@types/read_receipts';
-import { useCallback, useEffect } from 'react';
+import { useCallback, useEffect, useRef } from 'react';
 import {
   Membership,
   NotificationType,
@@ -29,6 +30,9 @@ import { roomToParentsAtom } from './roomToParents';
 import { useStateEventCallback } from '../../hooks/useStateEventCallback';
 import { useSyncState } from '../../hooks/useSyncState';
 import { useRoomsNotificationPreferencesContext } from '../../hooks/useRoomsNotificationPreferences';
+import { useRoomsListener } from '../../hooks/useRoomsListener';
+import { threadNotificationsAtom } from '../threadNotifications';
+import { getMutedThreads } from '../../utils/threadNotifications';
 
 export type RoomToUnreadAction =
   | {
@@ -169,11 +173,17 @@ export const roomToUnreadAtom = atom<RoomToUnread, [RoomToUnreadAction], undefin
 export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roomToUnreadAtom) => {
   const setUnreadAtom = useSetAtom(unreadAtom);
   const roomsNotificationPreferences = useRoomsNotificationPreferencesContext();
+  const threadNotifications = useAtomValue(threadNotificationsAtom);
+  // Latest thread-notification prefs for the SDK-event handlers below, read via a
+  // ref so changing prefs never re-attaches the (many) per-room listeners. The
+  // dedicated reset effect keyed on `threadNotifications` handles mute/unmute.
+  const threadNotificationsRef = useRef(threadNotifications);
+  threadNotificationsRef.current = threadNotifications;
 
   useEffect(() => {
     setUnreadAtom({
       type: 'RESET',
-      unreadInfos: getUnreadInfos(mx),
+      unreadInfos: getUnreadInfos(mx, threadNotificationsRef.current),
     });
   }, [mx, setUnreadAtom]);
 
@@ -187,7 +197,7 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
         ) {
           setUnreadAtom({
             type: 'RESET',
-            unreadInfos: getUnreadInfos(mx),
+            unreadInfos: getUnreadInfos(mx, threadNotificationsRef.current),
           });
         }
       },
@@ -204,6 +214,10 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
       data: IRoomTimelineData,
     ) => {
       if (!room || !data.liveEvent || room.isSpaceRoom() || !isNotificationEvent(mEvent)) return;
+      // Single-owner rule: thread replies drive the room badge via
+      // RoomEvent.UnreadNotifications below — ignore them here so the count is
+      // never double-driven / mis-attributed to the main timeline.
+      if (mEvent.threadRootId && mEvent.getId() !== mEvent.threadRootId) return;
       if (getNotificationType(mx, room.roomId) === NotificationType.Mute) {
         setUnreadAtom({
           type: 'DELETE',
@@ -213,7 +227,13 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
       }
 
       if (mEvent.getSender() === mx.getUserId()) return;
-      setUnreadAtom({ type: 'PUT', unreadInfo: getUnreadInfo(room) });
+      setUnreadAtom({
+        type: 'PUT',
+        unreadInfo: getUnreadInfo(
+          room,
+          getMutedThreads(threadNotificationsRef.current, room.roomId),
+        ),
+      });
     };
     mx.on(RoomEvent.Timeline, handleTimelineEvent);
     return () => {
@@ -246,10 +266,51 @@ export const useBindRoomToUnreadAtom = (mx: MatrixClient, unreadAtom: typeof roo
   useEffect(() => {
     setUnreadAtom({
       type: 'RESET',
-      unreadInfos: getUnreadInfos(mx),
+      unreadInfos: getUnreadInfos(mx, threadNotificationsRef.current),
     });
   }, [mx, setUnreadAtom, roomsNotificationPreferences]);
 
+  // Mute/unmute of a thread rewrites `threadNotificationsAtom`; recompute badges
+  // immediately so muted-thread subtraction takes effect without waiting for the
+  // next timeline / unread event (mirrors the notification-preferences reset).
+  useEffect(() => {
+    setUnreadAtom({
+      type: 'RESET',
+      unreadInfos: getUnreadInfos(mx, threadNotifications),
+    });
+  }, [mx, setUnreadAtom, threadNotifications]);
+
+  // RoomEvent.UnreadNotifications is emitted room-level only (never re-emitted
+  // client-side), so the main Timeline pathway misses thread-count changes and
+  // room badges lag. useRoomsListener appends the emitting Room as the final
+  // arg, making this a surgical per-room PUT (not a full RESET per emit) with
+  // muted-thread subtraction re-applied. Room-mute keeps its DELETE semantics.
+  useRoomsListener(
+    mx,
+    RoomEvent.UnreadNotifications,
+    useCallback(
+      (
+        _unreadNotifications: NotificationCount | undefined,
+        _threadId: string | undefined,
+        room: Room,
+      ) => {
+        if (room.isSpaceRoom()) return;
+        if (getNotificationType(mx, room.roomId) === NotificationType.Mute) {
+          setUnreadAtom({ type: 'DELETE', roomId: room.roomId });
+          return;
+        }
+        setUnreadAtom({
+          type: 'PUT',
+          unreadInfo: getUnreadInfo(
+            room,
+            getMutedThreads(threadNotificationsRef.current, room.roomId),
+          ),
+        });
+      },
+      [mx, setUnreadAtom],
+    ),
+  );
+
   useEffect(() => {
     const handleMembershipChange = (room: Room, membership: string) => {
       if (membership !== Membership.Join) {

src/app/state/threadNotifications.ts

@@ -0,0 +1,36 @@
+import { atom, useSetAtom } from 'jotai';
+import { ClientEvent, MatrixClient, MatrixEvent } from 'matrix-js-sdk';
+import { useEffect } from 'react';
+import { AccountDataEvent } from '../../types/matrix/accountData';
+import { ThreadNotificationsContent } from '../utils/threadNotifications';
+
+// Holds the parsed `io.lotus.thread_notifications` account data. Seeded and
+// kept in sync by `useBindThreadNotificationsAtom`.
+export const threadNotificationsAtom = atom<ThreadNotificationsContent>({});
+
+const readContent = (mx: MatrixClient): ThreadNotificationsContent =>
+  ((mx as any).getAccountData(AccountDataEvent.LotusThreadNotifications)?.getContent() as
+    | ThreadNotificationsContent
+    | undefined) ?? {};
+
+export const useBindThreadNotificationsAtom = (
+  mx: MatrixClient,
+  threadNotifications: typeof threadNotificationsAtom,
+) => {
+  const setContent = useSetAtom(threadNotifications);
+
+  useEffect(() => {
+    setContent(readContent(mx));
+
+    const handleAccountData = (event: MatrixEvent) => {
+      if (event.getType() === AccountDataEvent.LotusThreadNotifications) {
+        setContent(event.getContent<ThreadNotificationsContent>() ?? {});
+      }
+    };
+
+    mx.on(ClientEvent.AccountData, handleAccountData);
+    return () => {
+      mx.removeListener(ClientEvent.AccountData, handleAccountData);
+    };
+  }, [mx, setContent]);
+};