fix: MIDI item playback respects visual duration + scale flicker guard

app/static/js/app.jsx

@@ -5221,6 +5221,7 @@ const PianoRollTabEditor = ({ st, zoom, bpm, viewportWidth, onClose, onUpdateNot
       const ids = draggedNote.selectedNoteIds || [];
       const firstStart = draggedNote.firstStart;
       const notesBefore = notesBeforeDragRef.current;
+      if (!notesBefore) return;
       setNotes(prev => prev.map(n => {
         if (!ids.includes(n.id)) return n;
         const orig = notesBefore ? notesBefore.find(o => o.id === n.id) : null;
@@ -9428,10 +9429,12 @@ const App = () => {
         const bpmVal = parseInt(bpm) || 120;
         const secondsPerBeat = 60.0 / bpmVal;
         midiItems.forEach(item => {
+          const itemEndSec = item.startTime + (item.duration || 4);
           const notes = item.notes || [];
           notes.forEach(note => {
             // note start/duration is in beats (for MIDI items)
             const noteStartSec = item.startTime + (note.start_beat || 0) * secondsPerBeat;
+            if (noteStartSec >= itemEndSec) return; // skip notes outside item's visual duration
             const noteDurSec = (note.duration_beats || 1) * secondsPerBeat;
             const noteEndSec = noteStartSec + noteDurSec;
             if (offsetTime < noteEndSec) {