Live deltas from session A were attaching to session B after New Session
when events arrived without session_id — fallback used the newly focused
activeSessionId (#47709).
Pin unscoped stream events to the session that received message.start
(#48281). Also reset RAF-pending view staging on new/resume/create so a
stale background flush cannot repaint over the switched chat (#47743).
Co-authored-by: Ray <rayjun0412@gmail.com>
Co-authored-by: zapabob <1920071390@campus.ouj.ac.jp>