fix(cursor): advance on bass-only beats; make final beat reachable

CHANGELOG.md

@@ -6,6 +6,22 @@ Format: [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ---
 
+## [Unreleased]
+
+### Fixed
+
+- **Cursor now advances on bass-only (left-hand) beats** — `_svBuildBeatTimeline`
+  previously walked only the first staff of the first track, so beats that
+  only existed on the bass staff were missing from the tick index and the
+  playback cursor would stall or skip during left-hand-only passages. It now
+  collects beats from all staves of the first track and dedupes by tick
+  (treble preferred at shared positions).
+- **Final beat of a song is now reachable** — `_svSyncCursor`'s binary search
+  capped `hi` at `beats.length - 2`, making the last beat unreachable by the
+  search; it's now `beats.length - 1`.
+
+---
+
 ## [0.3.0] — 2026-07-08
 
 ### Added

screen.js

@@ -3201,15 +3201,16 @@ function createFactory() {
     }
 
     // ── Beat timeline (tick → Beat lookup) ────────────────────────
-    // Walks score.tracks[0].staves[0] only — the first staff of the
-    // first track is sufficient to build the absolute-tick index used
-    // for cursor sync.
+    // Collects beats from ALL staves of the first track so bass-only
+    // positions aren't skipped during cursor lookup, then dedupes by
+    // tick (treble preferred at shared positions).
 
     function _svBuildBeatTimeline(score) {
         const out = [];
         try {
-            trackLoop: for (const track of (score.tracks || [])) {
-                for (const staff of (track.staves || [])) {
+            const tracks = score.tracks || [];
+            if (tracks.length) {
+                for (const staff of (tracks[0].staves || [])) {
                     for (const bar of (staff.bars || [])) {
                         for (const voice of (bar.voices || [])) {
                             for (const beat of (voice.beats || [])) {
@@ -3222,12 +3223,22 @@ function createFactory() {
                             }
                         }
                     }
-                    break trackLoop; // first staff per track is sufficient; also exits trackLoop
                 }
             }
         } catch (_) {}
         out.sort((a, b) => a.start - b.start);
-        return out;
+        // Dedupe by tick: keep the first entry at each start tick so
+        // treble beats win when both staves share the same position
+        // (staff 0's beats are pushed first and sort is stable).
+        const deduped = [];
+        let prevStart = -1;
+        for (const entry of out) {
+            if (entry.start !== prevStart) {
+                deduped.push(entry);
+                prevStart = entry.start;
+            }
+        }
+        return deduped;
     }
 
     function _svFindBeatAtTick(tick) {
@@ -3287,7 +3298,7 @@ function createFactory() {
 
         // Binary search: largest i where beats[i].time <= currentTime.
         if (currentTime < beats[0].time) return;
-        let lo = 0, hi = beats.length - 2, idx = 0;
+        let lo = 0, hi = beats.length - 1, idx = 0;
         while (lo <= hi) {
             const mid = (lo + hi) >> 1;
             if (beats[mid].time <= currentTime) { idx = mid; lo = mid + 1; }