# Phase 8 Plan 2: Message List & Auto-Scroll Summary

**Built message display with auto-scroll behavior**

## Accomplishments

- Created formatTimestamp utility with relative time formatting (TDD RED-GREEN)
- Created JKChatMessage component with avatar, name, message, timestamp
- Created JKChatLoadingSpinner and JKChatEmptyState components
- Created JKChatMessageList with auto-scroll logic (TDD RED-GREEN for behavior)
- Integrated message list into JKSessionChatWindow
- Auto-scroll respects user manual scrolling with 50px threshold and 300ms debounce

## Files Created/Modified

**Created:**
- `jam-ui/src/utils/formatTimestamp.js` - Timestamp formatting utility
- `jam-ui/src/utils/__tests__/formatTimestamp.test.js` - Unit tests (6 tests)
- `jam-ui/src/components/client/chat/JKChatMessage.js` - Message display component
- `jam-ui/src/components/client/chat/JKChatLoadingSpinner.js` - Loading state component
- `jam-ui/src/components/client/chat/JKChatEmptyState.js` - Empty state component
- `jam-ui/src/components/client/chat/JKChatMessageList.js` - Message list with auto-scroll
- `jam-ui/src/components/client/chat/__tests__/JKChatMessageList.test.js` - Unit tests (3 tests)
- `jam-ui/src/setupTests.js` - Jest setup for @testing-library/jest-dom

**Modified:**
- `jam-ui/src/components/client/JKSessionChatWindow.js` - Integrated JKChatMessageList
- `jam-ui/package.json` - Added dayjs, @testing-library/react@12, @testing-library/jest-dom@5

## Decisions Made

**Timestamp formatting:**
- "Just now" for < 1 minute
- "X minutes ago" for < 1 hour
- "X hours ago" for < 24 hours
- "Yesterday" for 1 day old
- Day name (e.g., "Monday") for < 7 days
- MM/DD/YYYY for older messages

**Auto-scroll behavior:**
- 50px threshold for "at bottom" detection (balances sensitivity with UX)
- 300ms debounce for scroll detection (prevents jittery behavior)
- Smooth scrolling for better UX
- Manual scroll disables auto-scroll until user returns to bottom
- Auto-scroll triggers on message count change, not individual message objects

**Testing strategy:**
- TDD for formatTimestamp utility (6 tests, 100% coverage)
- TDD for JKChatMessageList behavior (3 tests, core functionality)
- Mock Element.prototype.scrollTo for JSDOM compatibility
- Installed @testing-library/react@12 (compatible with React 16)

**Component architecture:**
- React.memo on JKChatMessage for performance (prevents re-renders on list scroll)
- useCallback for all handlers to prevent unnecessary re-renders
- Inline styles for MVP (SCSS styling deferred to Plan 8.3)
- Stateless components for loading/empty states (no props needed)

## Issues Encountered

None - plan executed smoothly following TDD methodology.

## Test Results

**All chat-related tests passing:**
- formatTimestamp.test.js: 6 tests passed
- JKChatMessageList.test.js: 3 tests passed
- sessionChatSlice.test.js: 68 tests passed (pre-existing)
- **Total: 77 tests passed**

**Note:** Some pre-existing test failures in other slices (activeSessionSlice, mediaSlice, sessionUISlice) - unrelated to chat implementation.

## Next Phase Readiness

Ready for Plan 3 (Chat Button & Unread Badge):
- Message list component complete and tested
- Auto-scroll logic validated
- Empty and loading states implemented
- Timestamp formatting working
- All selectors available (selectUnreadCount, selectTotalUnreadCount)
- Redux state management complete from Phase 7

## Technical Notes

**Auto-scroll state machine:**
1. Initial state: isUserScrolling = false
2. User scrolls → isUserScrolling = true
3. After 300ms debounce → check if at bottom
4. If at bottom → isUserScrolling = false (re-enable auto-scroll)
5. If not at bottom → isUserScrolling stays true (keep disabled)

**Performance considerations:**
- React.memo on JKChatMessage prevents unnecessary re-renders
- useCallback on all event handlers
- Auto-scroll only triggers on messages.length change (not on every message object change)
- Smooth scrolling for better perceived performance
- 100ms delay on mount scroll to ensure DOM is rendered

**Deviation from plan:**
- None - all tasks executed as specified
- Followed strict TDD RED-GREEN-REFACTOR for Tasks 1 and 4
- Commit structure: 2 for Task 1 (RED + GREEN), 1 for Task 2, 1 for Task 3, 2 for Task 4 (RED + GREEN), 1 for Task 5 = 7 commits (within expected 7-9 range)
