---
phase: 02-backing-track-seek-controls
plan: 01
subsystem: ui
tags: [react, hooks, jamClient, native-integration, seek-controls, uat, async-await]

# Dependency graph
requires:
  - phase: 01-backing-track-playback-monitoring
    provides: formatTime utility, 500ms polling, currentTime and duration state
provides:
  - Functional seek slider with drag-to-position capability
  - currentPositionMs and durationMs state for slider calculations
  - handleSeek implementation using jamClient.SessionTrackSeekMs()
  - Edge case handling for end-of-track seeking
  - jamClient async/await pattern established
affects: [03-backing-track-finalization, media-player-ui]

# Tech tracking
tech-stack:
  added: []
  patterns: [optimistic UI updates, millisecond state for slider precision, SessionTrackSeekMs() integration, jamClientProxy async/await pattern, end-of-track reset]

key-files:
  created: [.planning/phases/02-backing-track-seek-controls/02-01-ISSUES.md]
  modified: [jam-ui/src/components/client/JKSessionBackingTrackPlayer.js, jam-ui/src/components/client/JKSessionScreen.js]

key-decisions:
  - "Separate state for milliseconds (slider calculations) vs formatted strings (display)"
  - "Optimistic UI update before jamClient call for responsive UX"
  - "Polling useEffect auto-corrects position drift within 500ms"
  - "jamClient must NOT be stored in Redux state - pass as prop instead"
  - "All jamClient methods in jam-ui return Promises - always use async/await"
  - "jamClient returns string values - always parseInt() before math operations"
  - "End-of-track position requires SessionStopPlay() reset before new playback"
  - "Defer UAT-003 (seek while paused) to Phase 3 - minor impact, likely jamClient limitation"

patterns-established:
  - "Slider state pattern: numeric state (currentPositionMs/durationMs) for calculations, formatted state (currentTime/duration) for display"
  - "Optimistic update pattern: Update local state immediately, let polling correct any drift"
  - "Native client seek integration: jamClient.SessionTrackSeekMs(positionMs) with error handling"
  - "jamClient integration pattern in jam-ui: Always async/await, always parseInt string returns, never store in Redux"
  - "End-of-track detection: position >= duration - 50ms, reset via SessionStopPlay() before SessionStartPlay()"

issues-created: [.planning/phases/02-backing-track-seek-controls/02-01-ISSUES.md (UAT-003)]

# Metrics
duration: 120min
completed: 2026-01-14
uat-issues-found: 4
uat-issues-resolved: 3
uat-issues-deferred: 1
---

# Phase 2 Plan 1: Backing Track Seek Controls Summary

**Completed:** 2026-01-14
**Duration:** ~120 min (including UAT and critical fixes)
**Plan:** 02-backing-track-seek-controls/02-01-PLAN.md

## Objective

Make the backing track seek bar functional by wiring it to real playback position and implementing drag-to-position capability using jamClient.SessionTrackSeekMs().

## Performance

- **Duration:** ~120 min (2 min plan execution + 118 min UAT and fixes)
- **Started:** 2026-01-13T18:08:39Z
- **Completed:** 2026-01-14
- **Tasks:** 3 planned + 4 UAT fixes
- **Files modified:** 2
- **Commits:** 13 total (3 feat + 5 fix + 4 docs + 1 refactor)

## Accomplishments

### Core Functionality (Tasks 1-3) ✅
- ✅ Added `currentPositionMs` and `durationMs` state for numeric slider calculations
- ✅ Wired slider to reflect live playback position via polling updates
- ✅ Implemented `handleSeek()` with jamClient.SessionTrackSeekMs() integration
- ✅ Slider functional during playback

### UAT Discoveries and Fixes ✅
During user acceptance testing, discovered and resolved **4 critical issues** that were blocking core functionality:

**UAT-001: Redux Immutability Violation** *(Blocker - RESOLVED)*
- **Issue:** Player wouldn't open - jamClient object stored in Redux state caused mutation detection error
- **Root cause:** Storing non-serializable jamClient object in Redux state at `activeSession.backingTrackData.jamClient.app`
- **Fix:** Removed jamClient from Redux state, pass as direct prop from JKSessionScreen component scope
- **Impact:** Established pattern for all future jamClient usage - never store in Redux
- **Commit:** `94d43fe9c`

**UAT-002: NaN Values and No Audio** *(Blocker - RESOLVED)*
- **Issue:** Time displays showed "NaN:NaN", slider jumped to middle, no audio playback
- **Root causes:**
  1. jamClient methods return Promises but code called them synchronously without `await`
  2. jamClient returns string values ('0', '228818') not numbers
  3. SessionStartPlay() missing required playback mode parameter (1)
- **Critical discovery:** jam-ui uses jamClientProxy with Proxy pattern where ALL methods return Promises (different from legacy web's synchronous bridge.invokeMethod())
- **Fix:** Added async/await for all jamClient calls, parseInt() for string conversion, SessionStartPlay(1) parameter
- **Impact:** Established async/await pattern for all jamClient usage in jam-ui
- **Commits:** `0561c8988`, `2a435806e`, `2c5ba2f4b`

**UAT-003: Seek While Paused Limitation** *(Minor - OPEN/DEFERRED)*
- **Issue:** Seeking while paused doesn't take effect until next play cycle
- **Impact:** Seek during playback works perfectly, seeking while paused requires workaround
- **Severity:** Minor - most users seek while playing, workaround exists (start play first)
- **Analysis:** Appears to be jamClient limitation where SessionTrackSeekMs() only registers during active playback
- **Status:** Documented in ISSUES.md, deferred to Phase 3 for investigation

**UAT-004: Seek to End While Paused Breaks Player** *(Blocker - RESOLVED)*
- **Issue:** Seeking to end while paused, then clicking play caused player to break (no audio, slider resets, play button stops working)
- **Root cause:** jamClient enters end-of-track state when position >= duration, calling SessionStartPlay() without reset causes undefined behavior
- **Fix:** Detect end position (within 50ms of duration), call SessionStopPlay() to reset jamClient state, seek to 0, then start playback normally
- **Impact:** Better UX - player resets to beginning when trying to play from end position
- **Commits:** `ba93f1a27`, `fc2b26355`, `0db622ad6`

**Enhancement: Logging Improvements**
- **Added:** `[BTP]` prefix to all console logs for easier filtering during debugging
- **Benefit:** Significantly improved debugging efficiency during UAT
- **Commit:** `3fb235240`

## Task Commits

### Plan Execution
1. **Task 1: Add state for position in milliseconds** - `209cefcf2` (feat)
2. **Task 2: Wire slider to reflect current position** - `491c0a68e` (feat)
3. **Task 3: Implement functional handleSeek** - `18afc9370` (feat)

### UAT Fixes
4. **UAT-001: Redux immutability fix** - `94d43fe9c` (fix)
5. **UAT-002: NaN defensive checks** - `0561c8988` (fix)
6. **UAT-002: Async/await implementation** - `2a435806e` (fix)
7. **UAT-002: String parsing and parameters** - `2c5ba2f4b` (fix)
8. **UAT-004: End-of-track edge case** - `ba93f1a27` (fix)

### Documentation & Refinement
9. **UAT-004 documentation** - `0db622ad6` (docs)
10. **UAT-004 resolution confirmation** - `fc2b26355` (docs)
11. **Logging prefix enhancement** - `3fb235240` (refactor)
12. **Initial plan completion** - `6e69c8974` (docs)
13. **Plan creation** - `2e312849a` (docs)

## Files Created/Modified

**Created:**
- `.planning/phases/02-backing-track-seek-controls/02-01-ISSUES.md` - UAT issue tracking document

**Modified:**
- `jam-ui/src/components/client/JKSessionBackingTrackPlayer.js` - Added position state, wired slider, implemented seek, fixed async issues, added edge case handling, added diagnostic logging
- `jam-ui/src/components/client/JKSessionScreen.js` - Removed jamClient from Redux dispatch, pass as direct prop instead

## Key Technical Discoveries

### 1. jamClient Architecture in jam-ui
**Critical finding:** jam-ui uses jamClientProxy with Proxy pattern where ALL methods return Promises

- Different from legacy web's synchronous `bridge.invokeMethod()` calls
- Implementation in `jamClientProxy.js`: `sendMessage()` returns `deferred.promise`
- `setupAsyncProxy()` wraps all method calls to return `sendWhenReady()` Promise
- **Implication:** Always use async/await when calling jamClient methods in jam-ui components

### 2. jamClient Return Values
- All methods return string values ('0', '228818') not numbers
- Must use `parseInt(value, 10) || 0` before math operations or comparisons
- Example: `SessionCurrrentPlayPosMs()` returns `'1502'` (string) not `1502` (number)

### 3. Redux State Serialization
- Cannot store non-serializable objects (like jamClient) in Redux state
- Redux Toolkit's immutability checks catch this in development mode
- Error message: "A state mutation was detected between dispatches, in the path 'activeSession.backingTrackData.jamClient.app'"
- **Solution:** Pass jamClient as prop through component tree, never store in Redux

### 4. End-of-Track State Handling
- jamClient enters special state when `position >= duration`
- Must call `SessionStopPlay()` to reset before starting new playback
- Detection threshold: within 50ms of duration to avoid false positives during normal seeking
- Without reset: Player enters broken state (no audio, UI becomes non-functional)

### 5. SessionStartPlay Parameter
- Requires playback mode parameter: `SessionStartPlay(1)` for normal playback
- Without parameter: Audio doesn't play (silent failure)
- Legacy web code consistently uses `SessionStartPlay(1)` or `SessionStartPlay(data.playbackMode)`

## Decisions Made

1. **Separate state for milliseconds vs formatted strings**
   - Slider uses numeric values (`currentPositionMs`, `durationMs`)
   - Display uses M:SS format (`currentTime`, `duration`)
   - Rationale: Clean separation of concerns, easier calculations

2. **Optimistic UI updates**
   - Update local state immediately before jamClient call
   - Provides responsive UX feel
   - 500ms polling auto-corrects any drift

3. **End-of-track reset to beginning**
   - When playing from end position, reset to 0:00 and start playback
   - Better UX than broken state or silent failure
   - Matches common media player behavior

4. **jamClient as prop, not Redux**
   - Pass directly as prop instead of storing in Redux
   - Follows React best practices for non-serializable data
   - Prevents Redux immutability violations

5. **Always async/await for jamClient in jam-ui**
   - Established pattern for all jam-ui components
   - Different from legacy web synchronous pattern
   - Prevents Promise object issues

6. **Always parseInt jamClient returns**
   - Established pattern for all numeric operations
   - Prevents NaN issues in calculations and comparisons
   - Defensive `|| 0` fallback for safety

7. **Defer UAT-003 to Phase 3**
   - Accept seeking while paused limitation
   - Minor impact (seek during playback works perfectly)
   - Most users seek while playing
   - Likely requires jamClient changes (deeper investigation needed)

## Deviations from Plan

**Plan:** "Works identically during playback and when paused (no special handling needed)"

**Reality:** Seeking while paused has limitations (UAT-003). Added special handling for end-of-track edge case (UAT-004).

**Reason:** Plan didn't account for jamClient async architecture and state management behavior.

## Issues Encountered

1. **Redux immutability violation** - jamClient in Redux state
2. **Async/await missing** - jamClient Proxy pattern not understood
3. **String vs number types** - jamClient return values needed parsing
4. **Missing API parameter** - SessionStartPlay(1) required
5. **End-of-track state** - jamClient requires reset before replay

**All resolved except UAT-003 (deferred)**

## Edge Cases Tested

✅ **Pass:** Seek while playing - audio jumps to position
✅ **Pass:** Seek to start - restarts from 0:00
✅ **Pass:** Seek to end - resets to beginning and plays
✅ **Pass:** Play → Stop → Seek → Play - works correctly
✅ **Pass:** Seek past end boundary - slider stays at max
⚠️ **Limitation:** Rapid seeks while paused - doesn't register (UAT-003)
⚠️ **Limitation:** Seek during initial load - doesn't register (UAT-003)

## Verification Results

**Core Functionality:**
- ✅ Slider starts at far left (0:00)
- ✅ Slider moves smoothly during playback
- ✅ Slider position matches time display
- ✅ Drag slider during playback - audio jumps to position
- ✅ Time display updates to match slider
- ✅ Seek to end resets to beginning (edge case handling)
- ✅ Seek to start works correctly
- ✅ Console clean (no errors during normal operation)

**Edge Cases:**
- ✅ Seek to end while paused - works (resets to beginning)
- ⚠️ Seek while paused - limitation documented (UAT-003)
- ✅ Multiple seeks while playing - works
- ✅ Play after stop - works
- ✅ Seek past end boundary - handled gracefully

## Known Limitations

**UAT-003: Seeking while paused doesn't register with jamClient**
- **Severity:** Minor
- **Workaround:** Start playback, then seek
- **Impact:** Seek during playback works perfectly, most users seek while playing
- **Status:** Documented, deferred to Phase 3
- **Likely cause:** jamClient limitation (SessionTrackSeekMs() only active during playback)

## Next Phase Readiness

✅ **Phase 3 (Backing Track Finalization) ready to proceed**

Phase 2 delivered fully functional seek controls with robust edge case handling. The player now has complete core functionality:
- ✅ Play/Pause/Stop controls
- ✅ Real-time position monitoring (500ms polling)
- ✅ Time display (current/duration in M:SS format)
- ✅ Functional seek slider with drag-to-position
- ✅ Edge case handling (end-of-track reset)
- ✅ jamClient async/await patterns established
- ✅ Redux state rules enforced

**Deferred to Phase 3:**
- Investigate and fix UAT-003 (seek while paused) or document as known limitation
- Comprehensive error handling (jamClient failures, file errors, network issues)
- Performance optimization (polling frequency tuning, cleanup on unmount)
- UI polish (loading states, smooth transitions, disabled states)
- Loop functionality testing
- Volume control testing

## Patterns Established for Future Work

### jamClient Integration Pattern (jam-ui)
```javascript
// ✅ CORRECT - jam-ui pattern
const handleAction = async () => {
  const value = await jamClient.SomeMethod();
  const numericValue = parseInt(value, 10) || 0;
  // ... use numericValue
};

// ❌ WRONG - Don't use synchronous calls
const value = jamClient.SomeMethod(); // Returns Promise, not value!

// ❌ WRONG - Don't store in Redux
dispatch(setState({ jamClient: jamClient })); // Immutability violation!
```

### End-of-Track Detection Pattern
```javascript
// Detect end position (within 50ms threshold)
if (currentPositionMs >= durationMs - 50 && durationMs > 0) {
  await jamClient.SessionStopPlay(); // Reset state
  await jamClient.SessionTrackSeekMs(0); // Seek to start
  // Now safe to start playback
  await jamClient.SessionStartPlay(1);
}
```

## Lessons Learned

1. **UAT is critical** - Discovered 4 major issues that unit tests wouldn't catch
2. **jamClient architecture differs from legacy** - Always use async/await in jam-ui components
3. **Redux immutability rules are strict** - Never store non-serializable objects in Redux state
4. **Edge cases matter** - End-of-track position requires special handling to prevent broken states
5. **Logging prefixes save time** - `[BTP]` prefix made debugging significantly faster
6. **Type safety matters** - jamClient string returns caused NaN issues until parseInt() added
7. **Plan assumptions may be wrong** - "Works identically when paused" assumption proven false by UAT
8. **API parameters matter** - SessionStartPlay(1) required, silent failure without it

---

*Phase: 02-backing-track-seek-controls*
*Plan: 01*
*Duration: ~120 min*
*Status: Complete*
*UAT Issues: 4 found, 3 resolved, 1 deferred*
