feat(editor): add measure signature readout

screen.html

@@ -49,6 +49,7 @@
             <svg id="editor-play-icon" class="w-3.5 h-3.5" fill="currentColor" viewBox="0 0 24 24"><path d="M8 5v14l11-7z"/></svg>
         </button>
         <span id="editor-time-display" class="text-xs text-gray-500 font-mono w-20">0:00 / 0:00</span>
+        <span id="editor-measure-display" class="text-xs text-gray-500 font-mono w-16" title="Current measure and time signature">M-- --</span>
         <!-- Loop selected bars on the 3D highway (drag the bottom beat bar to
              select a range first). Returns to this exact edit position on exit. -->
         <button id="editor-loop3d-btn" onclick="editorLoopIn3D()" class="px-3 py-1 bg-accent hover:bg-accent-light rounded text-xs font-medium disabled:opacity-50 disabled:cursor-not-allowed" disabled title="Select some notes or set a loop region first">▶ Loop in 3D</button>

screen.js

@@ -4681,6 +4681,16 @@ function updatePlayIcon() {
     }
 }
 
+function updateMeasureDisplay() {
+    const el = document.getElementById('editor-measure-display');
+    if (!el) return;
+    const selectedIdx = S.tempoMapMode ? S.tempoSel : -1;
+    const r = _editorMeasureSignatureReadoutPure(S.beats || [], S.cursorTime || 0, selectedIdx);
+    el.textContent = r.label;
+    el.title = r.measure === null
+        ? 'No measure grid available'
+        : `Measure ${r.measure}, time signature ${r.numerator}/${r.denominator}`;
+}
 function updateTimeDisplay() {
     const el = document.getElementById('editor-time-display');
     if (!el) return;
@@ -4690,6 +4700,7 @@ function updateTimeDisplay() {
         return m + ':' + String(s).padStart(2, '0');
     };
     el.textContent = fmt(S.cursorTime) + ' / ' + fmt(S.duration);
+    updateMeasureDisplay();
 }
 
 // ════════════════════════════════════════════════════════════════════
@@ -5973,6 +5984,42 @@ function _tempoResolvedMeasureIdx() {
     return measures[0].i;
 }
 
+
+/* @pure:measure-readout:start */
+function _editorMeasureSignatureReadoutPure(beats, time, selectedIdx) {
+    if (!Array.isArray(beats) || !beats.length) return { label: 'M-- --', measure: null, numerator: null, denominator: null };
+    let idx = Number.isInteger(selectedIdx) && selectedIdx >= 0 && selectedIdx < beats.length && beats[selectedIdx] && beats[selectedIdx].measure > 0
+        ? selectedIdx
+        : -1;
+    const t = Number.isFinite(Number(time)) ? Number(time) : 0;
+    if (idx < 0) {
+        for (let i = 0; i < beats.length; i++) {
+            const b = beats[i];
+            if (!b || b.measure <= 0) continue;
+            if ((Number(b.time) || 0) <= t + 1e-6) idx = i;
+            else break;
+        }
+    }
+    if (idx < 0) idx = beats.findIndex(b => b && b.measure > 0);
+    if (idx < 0) return { label: 'M-- --', measure: null, numerator: null, denominator: null };
+    const downbeat = beats[idx];
+    let nextIdx = beats.length;
+    for (let i = idx + 1; i < beats.length; i++) {
+        if (beats[i] && beats[i].measure > 0) { nextIdx = i; break; }
+    }
+    let numerator = Math.max(1, nextIdx - idx);
+    if (nextIdx === beats.length) {
+        let prevIdx = -1;
+        for (let i = idx - 1; i >= 0; i--) {
+            if (beats[i] && beats[i].measure > 0) { prevIdx = i; break; }
+        }
+        if (prevIdx >= 0) numerator = Math.max(1, idx - prevIdx);
+    }
+    const den = _tempoNormalizeDenominatorPure(downbeat.den);
+    const measure = downbeat.measure;
+    return { label: `M${measure} ${numerator}/${den}`, measure, numerator, denominator: den };
+}
+/* @pure:measure-readout:end */
 function updateBPMDisplay() {
     const el = document.getElementById('editor-bpm');
     if (!el || S.beats.length < 2) return;
@@ -6000,6 +6047,7 @@ function updateTempoSigDisplay() {
     }
     if (numEl) numEl.value = String(_tempoMeasureBeatCount(d));
     if (denEl) denEl.value = String(_tempoMeasureDenominator(d));
+    updateMeasureDisplay();
 }
 
 // Defer a `resizeCanvas` until layout has settled — used when a

tests/measure_readout.test.js

@@ -0,0 +1,66 @@
+'use strict';
+/*
+ * Measure/signature readout helper tests for screen.js.
+ *
+ * Run: node tests/measure_readout.test.js
+ */
+const fs = require('fs');
+const path = require('path');
+const assert = require('assert');
+
+const src = fs.readFileSync(path.join(__dirname, '..', 'screen.js'), 'utf8');
+const ts = src.match(/\/\* @pure:tempo-map-timesig:start \*\/[\s\S]*?\/\* @pure:tempo-map-timesig:end \*\//);
+const mr = src.match(/\/\* @pure:measure-readout:start \*\/[\s\S]*?\/\* @pure:measure-readout:end \*\//);
+if (!ts || !mr) {
+    console.error('FAIL: required pure blocks not found in screen.js');
+    process.exit(1);
+}
+
+const api = new Function(
+    '"use strict";' + ts[0] + '\n' + mr[0] + '\nreturn { _editorMeasureSignatureReadoutPure };'
+)();
+
+let pass = 0;
+let fail = 0;
+function t(name, fn) {
+    try { fn(); pass++; console.log('  ok   ' + name); }
+    catch (e) { fail++; console.error('  FAIL ' + name + ': ' + e.message); }
+}
+
+const beats = [
+    { time: 0, measure: 1, den: 4 },
+    { time: 1, measure: -1 },
+    { time: 2, measure: -1 },
+    { time: 3, measure: -1 },
+    { time: 4, measure: 2, den: 8 },
+    { time: 5, measure: -1 },
+    { time: 6, measure: -1 },
+    { time: 7, measure: -1 },
+    { time: 8, measure: 3 },
+    { time: 9, measure: -1 },
+    { time: 10, measure: -1 },
+    { time: 11, measure: -1 },
+];
+
+t('follows the cursor measure in note view', () => {
+    assert.deepStrictEqual(api._editorMeasureSignatureReadoutPure(beats, 4.25, -1), {
+        label: 'M2 4/8', measure: 2, numerator: 4, denominator: 8,
+    });
+});
+
+t('prefers selected tempo sync point over cursor time', () => {
+    assert.strictEqual(api._editorMeasureSignatureReadoutPure(beats, 1.2, 4).label, 'M2 4/8');
+});
+
+t('falls back to denominator 4 when no denominator is authored', () => {
+    assert.strictEqual(api._editorMeasureSignatureReadoutPure(beats, 8.1, -1).label, 'M3 4/4');
+});
+
+t('returns empty label shape when there is no downbeat grid', () => {
+    assert.deepStrictEqual(api._editorMeasureSignatureReadoutPure([{ time: 0, measure: -1 }], 0, -1), {
+        label: 'M-- --', measure: null, numerator: null, denominator: null,
+    });
+});
+
+console.log(`\n${pass} passed, ${fail} failed`);
+process.exit(fail ? 1 : 0);