fix: preserve playhead session position across dropdown switch

handleSwitchMidiItem now computes new currentTime from the delta
between old and new renderBeatOffset, keeping the playhead at
the same session-absolute time instead of resetting to bar 0.

app/static/js/app.jsx

@@ -4644,6 +4644,10 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     var match = allMidiItems.find(function(m) { return m.id === itemId; });
     if (!match) return;
     var trk = (activeTracks || []).find(function(t) { return t.id === match._trackId; });
+    var newBeatOff = (match.startTime / secondsPerBar) * timeSigNum;
+    var spb = 60.0 / (parseInt(bpm) || 120);
+    var deltaSec = (renderBeatOffset - newBeatOff) * spb;
+    var newTime = Math.max(0, (st.currentTime || 0) + deltaSec);
     setSubTabs(function(prev) {
       return prev.map(function(s) {
         if (s.id !== st.id) return s;
@@ -4656,7 +4660,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
           instrumentProgram: trk ? trk.instrumentProgram : undefined,
           instrumentName: trk ? trk.instrumentName : undefined,
           note_selection: [],
-          currentTime: 0
+          currentTime: newTime
         });
       });
     });