Root Cause:

The unread count clearing mechanism had a race condition when users sent messages during the debounce period. The LaunchedEffect was keyed on the entire messages list, which meant:

1. User receives message, unread divider appears, 3-second debounce timer starts
2. User sends message during this window, messages list changes
3. LaunchedEffect(messages) restarts, cancels the in-flight debounce timer
4. snapshotFlow starts fresh, but since the user isn't scrolling anymore, it doesn't emit a new value
5. No new debounce timer starts, unread count never clears until another message arrives

Changes Made:

1. Fixed race condition in UpdateUnreadCount

2. Simplified and corrected unread marking logic

3. Reduced debounce time

Improves responsiveness while still preventing database thrashing during scrolling