fix: playhead at bar 0 when opening MIDI item in session mode

- Removed st.currentTime >= 0 check from playhead rendering
- handleEditMidiInTab: currentTime = -beatOff * beatSec so phBeat=0
- handleSwitchMidiItem: same logic for dropdown switches
- Playhead draws at x=0 (bar 0) when item opens, moves right during
  playback from the initial negative position

app/static/js/app.jsx

@@ -4646,8 +4646,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     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);
+    var newTime = -newBeatOff * spb;
     setSubTabs(function(prev) {
       return prev.map(function(s) {
         if (s.id !== st.id) return s;
@@ -4986,7 +4985,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, activeTracks, onClos
     }
 
     // Draw playhead
-    if (st.currentTime !== undefined && st.currentTime !== null && st.currentTime >= 0) {
+    if (st.currentTime !== undefined && st.currentTime !== null) {
       const phBeat = renderBeatOffset + st.currentTime / (60.0 / (parseInt(bpm) || 120));
       const phX = phBeat * pixelsPerBeat;
       if (phX >= 0 && phX <= viewWidth) {
@@ -8287,6 +8286,10 @@ const App = () => {
     const tabLabel = `Piano Roll: ${midiItem.name || 'MIDI'}`;
     const ctx = getAudioContext();
     const silentBuffer = ctx.createBuffer(1, 128, ctx.sampleRate);
+    var bpmVal = parseInt(bpm) || 120;
+    var beatSec = 60.0 / bpmVal;
+    var secPerBar = beatSec * 4;
+    var beatOff = (midiItem.startTime / secPerBar) * 4;
     const newTab = {
       id: tabId,
       label: tabLabel,
@@ -8299,7 +8302,7 @@ const App = () => {
       buffer: silentBuffer,
       instrumentProgram: track.instrumentProgram,
       instrumentName: track.instrumentName,
-      currentTime: 0,
+      currentTime: -beatOff * beatSec,
       isPlaying: false,
       selectionStart: null,
       selectionEnd: null,