fix: pass channel+synth_engine to playNote in all MIDI playback paths

6 call sites were missing channel parameter, causing FluidSynth
to use ch=0 for all tracks -> program_change on channel 0
overwrote instrument across tracks during playback.

Fixes: startTrackPlayback, section sub-track MIDI, playMidiPreviewNote,
piano roll Alt+scroll preview, note click preview, keybed preview.

app/static/js/app.jsx

@@ -4842,8 +4842,14 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
           const playing = notes.filter(n =>
             currentBeat < n.start_beat && newBeat >= n.start_beat
           );
+          var pvTrk = activeTracks.find(function(t) { return t.id === st.trackId; });
+          var pvCh = pvTrk ? pvTrk.midiChannel : undefined;
+          if (pvCh === undefined && pvTrk) {
+            for (var pvi = 0; pvi < activeTracks.length; pvi++) { if (activeTracks[pvi].id === st.trackId) { pvCh = pvi >= 9 ? pvi + 1 : pvi; break; } }
+            if (pvCh === undefined || pvCh >= 16) pvCh = 0;
+          }
           playing.forEach(n => {
-            window.SonicSF.playNote(n.pitch, (n.velocity || 0.8) * 127, 200, ctx.currentTime, st.instrumentProgram, null);
+            window.SonicSF.playNote(n.pitch, (n.velocity || 0.8) * 127, 200, ctx.currentTime, st.instrumentProgram, null, pvCh, pvTrk ? pvTrk.synth_engine : undefined);
           });
         }
       }
@@ -5118,7 +5124,13 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     if (clickedNoteIdx !== -1 && !e.ctrlKey && !e.shiftKey && !e.altKey) {
       if (window.SonicSF) {
         const ctx = getAudioContext();
-        window.SonicSF.playNote(notes[clickedNoteIdx].pitch, 100, 300, ctx.currentTime, st.instrumentProgram, null);
+        var clTrk = activeTracks.find(function(t) { return t.id === st.trackId; });
+        var clCh = clTrk ? clTrk.midiChannel : undefined;
+        if (clCh === undefined && clTrk) {
+          for (var cli = 0; cli < activeTracks.length; cli++) { if (activeTracks[cli].id === st.trackId) { clCh = cli >= 9 ? cli + 1 : cli; break; } }
+          if (clCh === undefined || clCh >= 16) clCh = 0;
+        }
+        window.SonicSF.playNote(notes[clickedNoteIdx].pitch, 100, 300, ctx.currentTime, st.instrumentProgram, null, clCh, clTrk ? clTrk.synth_engine : undefined);
       }
     }
 
@@ -5640,6 +5652,13 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
   };
 
   const renderKeybed = () => {
+    var kbTrk = activeTracks.find(function(t) { return t.id === st.trackId; });
+    var kbCh = kbTrk ? kbTrk.midiChannel : undefined;
+    if (kbCh === undefined && kbTrk) {
+      for (var kbi = 0; kbi < activeTracks.length; kbi++) { if (activeTracks[kbi].id === st.trackId) { kbCh = kbi >= 9 ? kbi + 1 : kbi; break; } }
+      if (kbCh === undefined || kbCh >= 16) kbCh = 0;
+    }
+    var kbSynth = kbTrk ? kbTrk.synth_engine : undefined;
     const keys = [];
     for (let pitch = 127; pitch >= PITCH_START; pitch--) {
       const isBlack = [1, 3, 6, 8, 10].includes(pitch % 12);
@@ -5658,7 +5677,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
             keybedMouseDownRef.current = true;
             try {
               if (window.SonicSF) {
-                window.SonicSF.playNote(pitch, 100, 500, undefined, st.instrumentProgram, null);
+                window.SonicSF.playNote(pitch, 100, 500, undefined, st.instrumentProgram, null, kbCh, kbSynth);
               }
             } catch (err) {
               console.error('playNote error:', err);
@@ -5668,7 +5687,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
             if (keybedMouseDownRef.current) {
               try {
                 if (window.SonicSF) {
-                  window.SonicSF.playNote(pitch, 100, 200, undefined, st.instrumentProgram, null);
+                  window.SonicSF.playNote(pitch, 100, 200, undefined, st.instrumentProgram, null, kbCh, kbSynth);
                 }
               } catch (err) { console.error('playNote error:', err); }
             }
@@ -10114,13 +10133,22 @@ const App = () => {
     const track = activeTracks.find(t => t.id === trackId);
     const destNode = getOrCreateTrackNode(track, context);
     const program = track ? track.instrumentProgram : undefined;
+    var prevCh = track ? track.midiChannel : undefined;
+    if (prevCh === undefined && track) {
+      for (var pi = 0; pi < activeTracks.length; pi++) {
+        if (activeTracks[pi].id === track.id) { prevCh = pi >= 9 ? pi + 1 : pi; break; }
+      }
+      if (prevCh === undefined || prevCh >= 16) prevCh = 0;
+    }
     window.SonicSF.playNote(
       pitch,
       velocity,
       durationMs,
       context.currentTime,
       program,
-      destNode
+      destNode,
+      prevCh,
+      track ? track.synth_engine : undefined
     );
   };
 
@@ -10163,6 +10191,16 @@ const App = () => {
       // MIDI items playback
       const midiItems = track.midiItems || [];
       if ((track.type === 'MIDI' || midiItems.length > 0) && window.SonicSF) {
+        var trkCh = track.midiChannel;
+        if (trkCh === undefined) {
+          for (var tci = 0; tci < activeTracks.length; tci++) {
+            if (activeTracks[tci].id === track.id) {
+              trkCh = tci >= 9 ? tci + 1 : tci;
+              break;
+            }
+          }
+          if (trkCh === undefined || trkCh >= 16) trkCh = 0;
+        }
         const bpmVal = parseInt(bpm) || 120;
         const secondsPerBeat = 60.0 / bpmVal;
         midiItems.forEach(item => {
@@ -10186,7 +10224,9 @@ const App = () => {
                   durationMs,
                   startTime,
                   program,
-                  gainNode
+                  gainNode,
+                  trkCh,
+                  track.synth_engine
                 );
               } else {
                 const playOffset = offsetTime - noteStartSec;
@@ -10197,7 +10237,9 @@ const App = () => {
                   remainingDurMs,
                   context.currentTime,
                   program,
-                  gainNode
+                  gainNode,
+                  trkCh,
+                  track.synth_engine
                 );
               }
             }
@@ -10224,7 +10266,7 @@ const App = () => {
           const secTracks = sec.tracks || [];
           const hasSubSolo = secTracks.some(st => st.solo);
 
-          secTracks.forEach(subTrack => {
+          secTracks.forEach((subTrack, subIdx) => {
             const isSubPlayable = hasSubSolo ? subTrack.solo : !subTrack.muted;
             if (!isSubPlayable) return;
 
@@ -10283,6 +10325,11 @@ const App = () => {
                   if (noteStartMain >= secStart && noteStartMain < secEnd && offsetTime < noteEndMain) {
                     const notePlayEndMain = Math.min(noteEndMain, secEnd);
                     const program = subTrack.instrumentProgram !== undefined ? subTrack.instrumentProgram : 0;
+                    var subCh = subTrack.midiChannel;
+                    if (subCh === undefined) {
+                      subCh = subIdx >= 9 ? subIdx + 1 : subIdx;
+                    }
+                    if (subCh === undefined || subCh >= 16) subCh = 0;
 
                     if (offsetTime < noteStartMain) {
                       const delay = noteStartMain - offsetTime;
@@ -10294,7 +10341,9 @@ const App = () => {
                         playDurMs,
                         startTime,
                         program,
-                        subNode
+                        subNode,
+                        subCh,
+                        subTrack.synth_engine
                       );
                     } else {
                       const remainingDurMs = (notePlayEndMain - offsetTime) * 1000;
@@ -10304,7 +10353,9 @@ const App = () => {
                         remainingDurMs,
                         context.currentTime,
                         program,
-                        subNode
+                        subNode,
+                        subCh,
+                        subTrack.synth_engine
                       );
                     }
                   }

wiki.md

@@ -473,3 +473,9 @@
 - **Các file ảnh hưởng:** `app/static/js/app.jsx` (verify lines 4642-4667, 5905, 6763-6807, 8422-8424), `app/core/render_engine.py` (verify lines 95-115)
 - **Ghi chú/Test (nếu có):** No code changes needed. All 4 rules confirmed working.
 ---
+
+### [2026-07-27 20:37] Task: Fix cross-track instrument interference (missed channel param)
+- **Tóm tắt thay đổi:** Sửa 6 vị trí gọi `SonicSF.playNote()` thiếu tham số `channel` + `synthEngine`, khiến FluidSynth mặc định `ch=0` cho mọi track → `program_change` trên channel 0 đè lên nhau giữa các track. Fix: thêm channel computation (từ `track.midiChannel` hoặc track index) + truyền `synth_engine` vào `playNote` tại `startTrackPlayback`, section sub-track MIDI, `playMidiPreviewNote`, piano roll Alt+scroll preview, note click preview, keybed onMouseDown/onMouseEnter. `setTrackInstrumentWithProgram` đã filter đúng `trackId` — lỗi không nằm ở logic set instrument mà ở playback.
+- **Các file ảnh hưởng:** `app/static/js/app.jsx` (lines 4846, 5121, 5661-5671, 10117-10124, 10179-10201, 10285-10307)
+- **Ghi chú/Test (nếu có):** `npm run build` pass. Cần test: main session play 2 tracks MIDI khác instrument → mỗi track giữ instrument riêng. Piano Roll preview note → không ảnh hưởng track khác.
+---