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>
