fix(notifications): safe thread receipts on mark-read (fixes read-receipt regression)

The prior thread-receipt change (8192da5a) broke read receipts globally. Exact
cause: markAsRead used `thread.lastReply() ?? thread.rootEvent`. When a thread's
replies weren't loaded (lastReply() null — common on room open), it sent a
receipt for the thread ROOT. Since roots are "in the main timeline",
threadIdForReceipt() makes that a MAIN receipt at an old event; when the root
isn't in the loaded timeline the SDK's backward-guard falls back to timestamp
and applies it, moving the main read receipt onto an event we don't have, so
getEventReadUpTo() returns null and roomHaveUnread() reports the room unread —
re-broken on every mark-read, amplified by the bulk mark-all-orphan-rooms-read
callers.

Fix: main unthreaded receipt unchanged; the thread loop now sends a threaded
receipt ONLY for a genuine loaded thread reply (thread.lastReply()), never the
root — if replies aren't loaded, skip. New notifications.test.ts locks the
regression (null lastReply → no root receipt) + the main/threaded/no-op cases.

Gates: tsc/eslint/prettier clean, build OK, 672 tests (7 new).

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

src/app/utils/notifications.test.ts

@@ -0,0 +1,126 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { NotificationCountType, ReceiptType } from 'matrix-js-sdk';
+import { markAsRead } from './notifications';
+
+// markAsRead sends an unthreaded read receipt at the latest main-timeline event,
+// plus a THREADED receipt at each unread thread's latest loaded reply. The
+// regression these tests guard against: a thread whose replies aren't loaded
+// (lastReply() === null) must NOT produce a receipt for the thread root — that
+// resolves to a MAIN receipt at an old event and permanently unreads the room.
+
+type ReceiptCall = { eventId: string; receiptType: ReceiptType; unthreaded?: boolean };
+
+const evt = (id: string, sending = false) => ({ getId: () => id, isSending: () => sending }) as any;
+
+const thread = (id: string, lastReply: any) => ({ id, lastReply: () => lastReply }) as any;
+
+type RoomOpts = {
+  timeline?: any[];
+  readUpTo?: string | null;
+  threads?: any[];
+  threadUnread?: Record<string, number>;
+};
+
+const setup = (opts: RoomOpts) => {
+  const calls: ReceiptCall[] = [];
+  const room = {
+    getLiveTimeline: () => ({ getEvents: () => opts.timeline ?? [] }),
+    getEventReadUpTo: () => opts.readUpTo ?? null,
+    getThreads: () => opts.threads ?? [],
+    getThreadUnreadNotificationCount: (threadId: string, _type: NotificationCountType) =>
+      opts.threadUnread?.[threadId] ?? 0,
+  };
+  const mx = {
+    getRoom: () => room,
+    getUserId: () => '@me:server',
+    sendReadReceipt: async (event: any, receiptType: ReceiptType, unthreaded?: boolean) => {
+      calls.push({ eventId: event.getId(), receiptType, unthreaded });
+      return {};
+    },
+  } as any;
+  return { mx, calls };
+};
+
+test('main timeline: unthreaded receipt at the latest event', async () => {
+  const { mx, calls } = setup({ timeline: [evt('a'), evt('b'), evt('c')], readUpTo: 'a' });
+  await markAsRead(mx, '!r:server', false);
+  assert.equal(calls.length, 1);
+  assert.deepEqual(calls[0], { eventId: 'c', receiptType: ReceiptType.Read, unthreaded: true });
+});
+
+test('REGRESSION: an unread thread with unloaded replies (lastReply null) sends NO root receipt', async () => {
+  const t = thread('$root', null); // replies not loaded
+  const { mx, calls } = setup({
+    timeline: [evt('a'), evt('b')],
+    readUpTo: 'a',
+    threads: [t],
+    threadUnread: { $root: 3 },
+  });
+  await markAsRead(mx, '!r:server', false);
+  // Only the main unthreaded receipt — never a receipt for the thread root.
+  assert.equal(calls.length, 1);
+  assert.equal(calls[0].eventId, 'b');
+  assert.equal(calls[0].unthreaded, true);
+  assert.ok(!calls.some((c) => c.eventId === '$root'));
+});
+
+test('unread thread with a loaded reply sends a threaded receipt at that reply', async () => {
+  const t = thread('$root', evt('$reply'));
+  const { mx, calls } = setup({
+    timeline: [evt('a'), evt('b')],
+    readUpTo: 'a',
+    threads: [t],
+    threadUnread: { $root: 1 },
+  });
+  await markAsRead(mx, '!r:server', false);
+  const main = calls.find((c) => c.eventId === 'b');
+  const threaded = calls.find((c) => c.eventId === '$reply');
+  assert.ok(main && main.unthreaded === true);
+  assert.ok(threaded && threaded.unthreaded === false);
+  assert.equal(calls.length, 2);
+});
+
+test('main already read but a thread is unread: no main receipt, threaded receipt only', async () => {
+  const t = thread('$root', evt('$reply'));
+  const { mx, calls } = setup({
+    timeline: [evt('a'), evt('b')],
+    readUpTo: 'b', // latest main event already read → getLatestValidEvent() null
+    threads: [t],
+    threadUnread: { $root: 2 },
+  });
+  await markAsRead(mx, '!r:server', false);
+  assert.equal(calls.length, 1);
+  assert.equal(calls[0].eventId, '$reply');
+  assert.equal(calls[0].unthreaded, false);
+});
+
+test('everything read: no receipts sent', async () => {
+  const t = thread('$root', evt('$reply'));
+  const { mx, calls } = setup({
+    timeline: [evt('a'), evt('b')],
+    readUpTo: 'b',
+    threads: [t],
+    threadUnread: { $root: 0 }, // thread read too
+  });
+  await markAsRead(mx, '!r:server', false);
+  assert.equal(calls.length, 0);
+});
+
+test('sending thread reply is skipped', async () => {
+  const t = thread('$root', evt('$reply', true)); // isSending → skip
+  const { mx, calls } = setup({
+    timeline: [evt('a'), evt('b')],
+    readUpTo: 'b',
+    threads: [t],
+    threadUnread: { $root: 1 },
+  });
+  await markAsRead(mx, '!r:server', false);
+  assert.equal(calls.length, 0);
+});
+
+test('private receipt flag uses ReadPrivate', async () => {
+  const { mx, calls } = setup({ timeline: [evt('a'), evt('b')], readUpTo: 'a' });
+  await markAsRead(mx, '!r:server', true);
+  assert.equal(calls[0].receiptType, ReceiptType.ReadPrivate);
+});