Problem: After removing autoscroll, last read message wasn't visible on initial load
Solution:
- ✨ NEW: _positionToMessage() - instant viewport positioning WITHOUT scroll animation
- Sets scrollTop BEFORE user sees chat (no visible scrolling)
- Centers last read message with sticky header offset (96px)
- Falls back to bottom position for new chats
Difference:
- ❌ Old: render → scrollIntoView() → visible scroll animation
- ✅ New: render → scrollTop = X → instant positioning
Result:
- Chat opens IMMEDIATELY showing last read message
- NO visible scrolling or animation
- Telegram-like behavior: position is set, not scrolled to
- Natural user experience