docs(27): complete Backing Track Sync phase

- Phase 27 verified: 2/2 must-haves confirmed
- BT-01: openBackingTrack action enables session screen sync
- BT-02: ignore flag prevents unmount warnings
- ROADMAP.md, STATE.md, REQUIREMENTS.md updated

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.planning/REQUIREMENTS.md

@@ -16,8 +16,8 @@ Requirements for polishing media player features. Each maps to roadmap phases.
 
 ### Backing Track Fixes
 
-- [ ] **BT-01**: Use openBackingTrack() action to enable track sync to session screen
+- [x] **BT-01**: Use openBackingTrack() action to enable track sync to session screen ✓
-- [ ] **BT-02**: Add ignore flag to duration fetch useEffect
+- [x] **BT-02**: Add ignore flag to duration fetch useEffect ✓
 
 ### Metronome Fixes
 
@@ -50,8 +50,8 @@ Explicitly excluded for this milestone.
 | JT-02 | Phase 26 | Complete |
 | JT-03 | Phase 26 | Complete |
 | JT-04 | Phase 26 | Complete |
-| BT-01 | Phase 27 | Pending |
+| BT-01 | Phase 27 | Complete |
-| BT-02 | Phase 27 | Pending |
+| BT-02 | Phase 27 | Complete |
 | MET-01 | Phase 28 | Pending |
 | MET-02 | Phase 28 | Pending |
 

.planning/ROADMAP.md

@@ -17,7 +17,7 @@ v1.6 addresses usability issues reported in three media features: JamTrack (load
 ## Phases
 
 - [x] **Phase 26: JamTrack Polish** - Fix loading sequence, sizing, navigation, and cleanup ✓
-- [ ] **Phase 27: Backing Track Sync** - Enable track sync and async cleanup
+- [x] **Phase 27: Backing Track Sync** - Enable track sync and async cleanup ✓
 - [ ] **Phase 28: Metronome Responsiveness** - Debounce controls and cleanup timers
 
 ## Phase Details
@@ -49,7 +49,7 @@ Plans:
 **Plans**: 1 plan
 
 Plans:
-- [ ] 27-01-PLAN.md - Use openBackingTrack action and add async cleanup
+- [x] 27-01-PLAN.md - Use openBackingTrack action and add async cleanup ✓
 
 ### Phase 28: Metronome Responsiveness
 **Goal**: Metronome controls respond smoothly to user input
@@ -71,7 +71,7 @@ Plans:
 | Phase | Milestone | Plans Complete | Status | Completed |
 |-------|-----------|----------------|--------|-----------|
 | 26. JamTrack Polish | v1.6 | 4/4 | Complete | 2026-02-25 |
-| 27. Backing Track Sync | v1.6 | 0/1 | Planned | - |
+| 27. Backing Track Sync | v1.6 | 1/1 | Complete | 2026-02-26 |
 | 28. Metronome Responsiveness | v1.6 | 0/TBD | Not started | - |
 
 ---

.planning/phases/27-backing-track-sync/27-VERIFICATION.md

@@ -0,0 +1,88 @@
+---
+phase: 27-backing-track-sync
+verified: 2026-02-26T11:53:24Z
+status: passed
+score: 2/2 must-haves verified
+---
+
+# Phase 27: Backing Track Sync Verification Report
+
+**Phase Goal:** Backing Track appears in session screen when opened
+**Verified:** 2026-02-26T11:53:24Z
+**Status:** passed
+**Re-verification:** No - initial verification
+
+## Goal Achievement
+
+### Observable Truths
+
+| # | Truth | Status | Evidence |
+|---|-------|--------|----------|
+| 1 | Opening a backing track file shows the track in session screen (not just popup) | VERIFIED | `handleBackingTrackSelected` at line 1137 calls `await openBackingTrack(result.file)` which triggers `syncTracksToServer` in useMediaActions hook |
+| 2 | No "state update on unmounted component" warnings when closing backing track quickly | VERIFIED | JKSessionBackingTrackPlayer.js useEffect at line 90 has `let ignore = false` declaration and cleanup function at line 155-157 sets `ignore = true` |
+
+**Score:** 2/2 truths verified
+
+### Required Artifacts
+
+| Artifact | Expected | Status | Details |
+|----------|----------|--------|---------|
+| `jam-ui/src/components/client/JKSessionScreen.js` | handleBackingTrackSelected using openBackingTrack action | VERIFIED | Line 1144: `await openBackingTrack(result.file);` - uses action from useMediaActions (imported line 18, destructured line 153) |
+| `jam-ui/src/components/client/JKSessionBackingTrackPlayer.js` | Duration fetch useEffect with cleanup | VERIFIED | Lines 90-158: useEffect with `let ignore = false` (line 91), async checks, and cleanup `return () => { ignore = true; }` (lines 155-157) |
+
+### Key Link Verification
+
+| From | To | Via | Status | Details |
+|------|-----|-----|--------|---------|
+| JKSessionScreen.js handleBackingTrackSelected | useMediaActions openBackingTrack | async await call | VERIFIED | Line 1144: `await openBackingTrack(result.file)` - openBackingTrack destructured from useMediaActions() at line 153 |
+| JKSessionBackingTrackPlayer.js useEffect | cleanup function | return statement | VERIFIED | Lines 155-157: `return () => { ignore = true; };` |
+| openBackingTrack hook function | syncTracksToServer | dispatch call | VERIFIED | useMediaActions.js line 60: `dispatch(syncTracksToServer(sessionId, jamClient))` |
+
+### Requirements Coverage
+
+| Requirement | Status | Blocking Issue |
+|-------------|--------|----------------|
+| BT-01: Use openBackingTrack() action to enable track sync to session screen | SATISFIED | None |
+| BT-02: Add ignore flag to duration fetch useEffect | SATISFIED | None |
+
+### Anti-Patterns Found
+
+| File | Line | Pattern | Severity | Impact |
+|------|------|---------|----------|--------|
+| JKSessionScreen.js | 1158 | TODO comment about native window | Info | Pre-existing, not related to Phase 27 changes |
+
+Note: JKSessionBackingTrackPlayer.js has no TODO/FIXME patterns.
+
+### Human Verification Required
+
+#### 1. Visual Verification: Backing Track in Session Screen
+**Test:** Open a backing track file in session
+**Expected:** Backing track appears in the session screen track list (not just in popup)
+**Why human:** Requires running app with native client to verify visual rendering
+
+#### 2. Console Warning Check
+**Test:** Open backing track popup, then close it immediately (within 1-2 seconds)
+**Expected:** No React warning "Can't perform a React state update on an unmounted component"
+**Why human:** Requires running app and monitoring browser console during rapid open/close
+
+### Summary
+
+All must-haves have been verified in the codebase:
+
+1. **Backing track sync:** The `handleBackingTrackSelected` function now uses `openBackingTrack(result.file)` action from useMediaActions hook. This action:
+   - Calls `openBackingTrackThunk` which invokes `jamClient.SessionOpenBackingTrackFile`
+   - Sets `mediaSummary.backingTrackOpen = true`
+   - Calls `syncTracksToServer(sessionId, jamClient)` to sync track to session screen
+
+2. **Async cleanup:** The duration fetch useEffect in JKSessionBackingTrackPlayer.js implements the ignore flag pattern:
+   - `let ignore = false` at useEffect start (line 91)
+   - `if (ignore) return` checks before async operation (line 104) and after (line 119)
+   - `if (!ignore)` guard in catch block (line 143)
+   - Cleanup function `return () => { ignore = true; }` (lines 155-157)
+
+The direct `jamClient.SessionOpenBackingTrackFile` call has been removed from handleBackingTrackSelected (grep confirms no matches).
+
+---
+
+*Verified: 2026-02-26T11:53:24Z*
+*Verifier: Claude (gsd-verifier)*