Improve chat typing responsiveness in long sessions

[hamzamerzic]

Summary

• isolate live assistant source selection and rendering from composer-only updates
• use native textarea content sizing when supported while keeping the existing fallback
• progressively render drawer chat rows as the user scrolls

Why

Draft keystrokes currently rerun active-answer normalization and rendering, synchronously persist the same draft twice, force textarea layout measurement, and pay layout cost for every chat row mounted in the drawer. Long live responses and large histories can therefore make typing miss multiple frames.

The change gives these interactions separate ownership boundaries without changing chat or drawer behavior. The drawer remains one continuous history with no “recent” section or “show older” control.

In a long active chat, median input work fell from 42.8 ms to 13.3 ms and p95 from 110.8 ms to 20 ms. In a settled chat, median input work fell from 5.4 ms to 3.5 ms. Initial drawer descendants fell from about 2,883 to 604.

Validation

• npm test (1,877 tests)
• npm run build
• browser benchmark on settled and long active chats
• desktop and phone-width checks for composer growth and progressive drawer scrolling