Drum machine · mimo-v2.5

Let me build a 16-step drum sequencer in a single index.html file. Then verify with browser tools.

Design:
- 4 rows: Kick, Snare, Hi-Hat, Clap
- 16 steps each, toggleable
- Default pattern: kick on steps 0,4,8,12; snare on 4,12; hihat every other; clap on 8 maybe- Play, Stop, Clear buttons, BPM slider 60-180 (and number input)
- Web Audio API synthesis: kick = sine osc with pitch envelope; snare = noise + tone; hihat = filtered noise short; clap = noise bursts- Scheduling: lookahead scheduler with setInterval, schedule ahead0.1s, currentStep, nextNoteTime
- Highlight current step: draw queue based on scheduled time
- Audio unlock: AudioContext resumed on first user gesture (Play click or any click)
- Stop cancels future hits: we can't cancel scheduled notes easily... We can track scheduled sources and stop them, or simply stop the scheduler and let already-scheduled short sounds finish. Better: keep a list of scheduled nodes with their stop times and call stop(0) on them on Stop. Also use a "generation" counter so callbacks from setTimeout are ignored.

Let me write it carefully.

Scheduling approach:
```js
let audioCtx = null;
let isPlaying = false;
let currentStep = 0; // next step to schedule
let nextNoteTime = 0;
let timerId = null;
const lookahead = 25; // ms
const scheduleAhead = 0.1; // s
const drawQueue = []; // {step, time}
```

scheduler():
```js
while (nextNoteTime < audioCtx.currentTime + scheduleAhead) {
  scheduleStep(currentStep, nextNoteTime);
  drawQueue.push({step: currentStep, time: nextNoteTime});
  nextNoteTime += stepDur();
  currentStep = (currentStep + 1) % 16;
}
```

stepDur = 60 / bpm / 4.

BPM change affects stepDur on next scheduling — no duplicate sequences since we only have one scheduler loop. Good.

Playhead highlight: requestAnimationFrame draw loop that pops drawQueue entries whose time <= currentTime, sets highlight.

Stop: clear interval, clear drawQueue, stop all active scheduled sources, set currentStep = 0, remove highlight.

Voices:

Kick: osc sine, freq 150 -> 50 exponential ramp over 0.12s, gain env 1 -> 0.001 over 0.4s. Plus a click via short square? Keep simple.

Snare: white noise buffer through highpass, gain env 0.3s; plus a triangle osc at 180Hz for body, decay 0.1s.

Hi-hat: white noise through highpass 7000, gain env very short (0.05 open / 0.03 closed). I'll use closed hat always, 0.045s.

Clap: noise through bandpass 1000-1500, with 3 short bursts (0.01 apart) plus a tail. Simpler: multiple gain envelopes on same noise source? Use3 noise sources with staggered start times.

Noise buffer: create once per AudioContext (1 second of white noise), reuse.

Track active sources for stop: push {node, stopTime}; on stop, iterate and call node.stop() with try/catch, only those scheduled in future. Actually calling stop() twice throws; use try/catch. Also disconnect after ended.

Track scheduled nodes via a Set, remove onended.

UI: dark theme, grid with step buttons, beat groupings (4 columns with gap), current step highlight ring, active steps filled. Row labels. Also maybe a highlight header showing step numbers.

Accessibility: buttons with aria-pressed.

Audio unlock: on first interaction (pointerdown anywhere or Play), create/resume context. Show a small "Click Play to enable audio" hint that disappears.

Let me write the HTML.

Layout: 
- header: title, transport (Play, Stop, Clear), BPM control (range + number display)
- grid: label column + 16 buttons. Use CSS grid with repeat.

Beat groups: add margin-right on steps where (i+1)%4===0, and slightly larger gap.

Let me write the code now.

For pattern state: a 2D array steps[4][16] booleans. The DOM buttons reflect state. "Starting again uses the currently displayed pattern" — reading state at schedule time.

Also add keyboard? Not needed.

Let me also add a volume/master gain with a limiter (DynamicsCompressor) to avoid clipping.

Write file.

Colors: row colors — kick orange/red, snare red/pink, hihat yellow, clap purple/blue.

Let me write.

```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>16-Step Drum Sequencer</title>
<style>...</style>
</head>
<body>
...
<script>...</script>
</body>
</html>
```

CSS: use CSS variables, grid layout.

Grid markup:
```html
<div class="seq" id="seq"></div>
```
Build rows in JS: for each track, a label div + 16 buttons. Use CSS grid with `grid-template-columns: 110px repeat(16, 1fr)` and gap. But beat groupings need different gaps — use per-button margin.

Actually simpler: each row is a flex container: label (fixed width) + steps container of 16 buttons with gap, extra margin every 4.

I'll do:
```html
<div class="row">
  <div class="row-label">Kick</div>
  <div class="row-steps">
    <button class="step" data-track="0" data-step="0"></button>
    ...
    <div class="beat-gap"></div>  <!-- no, use nth-child margins -->
  </div>
</div>
```
Use `.step:nth-child(4n) { margin-right: 10px; }` and `.step:nth-child(4n+1) { margin-left: 0 }` — first has none anyway.

Step button size: aspect-ratio 1, flex1, min-width 0.

Also a playhead indicator row on top: 16 small markers. And on the step buttons, `.step.current` gets an outline/glow.

Mobile: allow horizontal scroll.

Now JS.

```js
const TRACKS = [
  {name:'Kick', color:'#ff7043', synth: playKick},
  {name:'Snare', ...},
  {name:'Hi-Hat', ...},
  {name:'Clap', ...},
];
const STEPS = 16;
const state = TRACKS.map(() => new Array(STEPS).fill(false));
// default pattern
[0,4,8,12].forEach(i=>state[0][i]=true);
[4,12].forEach(i=>state[1][i]=true);
for(let i=0;i<16;i+=2) state[2][i]=true;
state[3][10]=true; // clap offbeat
```

Hmm "simple audible pattern": kick 0,4,8,12; snare 4,12; hats every 8th; clap on 12? Let's put clap on 12 along with snare for a layered backbeat, or step 10. I'll do clap on 4 and 12 too? That's layered with snare making a thicker backbeat — nice. Actually let's do clap on 12 only for variety. Keep it: clap [12].

Audio:

```js
let ctx=null, master=null, noiseBuf=null;
function ensureAudio(){
  if(!ctx){
    ctx = new (window.AudioContext||window.webkitAudioContext)();
    master = ctx.createGain(); master.gain.value=0.9;
    const comp = ctx.createDynamicsCompressor();
    comp.threshold.value=-12; comp.ratio.value=4; comp.attack.value=0.003; comp.release.value=0.25;
    master.connect(comp); comp.connect(ctx.destination);
    noiseBuf = makeNoise(ctx);
  }
  if(ctx.state === 'suspended') ctx.resume();
}
```

makeNoise: 2s buffer of white noise.

Voices register sources:

```js
const liveSources = new Set();
function track(node){
  liveSources.add(node);
  node.addEventListener('ended', ()=>liveSources.delete(node));
}
function killAll(){
  liveSources.forEach(n=>{ try{n.stop();}catch(e){} });
  liveSources.clear();
}
```

Note: gain nodes not stopped; osc/noise buffer sources stopped. Also they're connected to master — when stopped, they end. Good.

playKick(t):
```js
const o=ctx.createOscillator(), g=ctx.createGain();
o.type='sine';
o.frequency.setValueAtTime(160,t);
o.frequency.exponentialRampToValueAtTime(48,t+0.12);
g.gain.setValueAtTime(0.0001,t);
g.gain.exponentialRampToValueAtTime(1.0,t+0.005);
g.gain.exponentialRampToValueAtTime(0.0001,t+0.35);
o.connect(g); g.connect(master);
o.start(t); o.stop(t+0.4); track(o);
// click
```
Add a click: short noise burst through highpass? Optional. Add small "beater" click: 
```js
const c=ctx.createOscillator(); c.type='triangle'; ... 
```
Keep it simple, skip.

playSnare(t):
```js
const noise = ctx.createBufferSource(); noise.buffer=noiseBuf; noise.loop=true;
const hp = ctx.createBiquadFilter(); hp.type='highpass'; hp.frequency.value=1400;
const g = ctx.createGain();
g.gain.setValueAtTime(0.9,t); g.gain.exponentialRampToValueAtTime(0.001,t+0.18);
noise.connect(hp); hp.connect(g); g.connect(master);
noise.start(t); noise.stop(t+0.2); track(noise);
// body
const o=ctx.createOscillator(); o.type='triangle'; o.frequency.setValueAtTime(200,t); o.frequency.exponentialRampToValueAtTime(120,t+0.08);
const og=ctx.createGain(); og.gain.setValueAtTime(0.5,t); og.gain.exponentialRampToValueAtTime(0.001,t+0.1);
o.connect(og); og.connect(master); o.start(t); o.stop(t+0.12); track(o);
```

playHat(t):
```js
const n=ctx.createBufferSource(); n.buffer=noiseBuf;
const hp=ctx.createBiquadFilter(); hp.type='highpass'; hp.frequency.value=7500;
const bp=ctx.createBiquadFilter(); bp.type='bandpass'; bp.frequency.value=10000; bp.Q.value=0.7;
const g=ctx.createGain(); g.gain.setValueAtTime(0.5,t); g.gain.exponentialRampToValueAtTime(0.001,t+0.05);
n.connect(hp); hp.connect(bp); bp.connect(g); g.connect(master);
n.start(t); n.stop(t+0.06); track(n);
```

playClap(t): three bursts```js
const n=ctx.createBufferSource(); n.buffer=noiseBuf;
const bp=ctx.createBiquadFilter(); bp.type='bandpass'; bp.frequency.value=1600; bp.Q.value=1.2;
const g=ctx.createGain();
g.gain.setValueAtTime(0.0001,t);
[0,0.012,0.024].forEach((off,i)=>{
  g.gain.setValueAtTime(0.8,t+off);
  g.gain.exponentialRampToValueAtTime(0.05,t+off+0.01);
});
g.gain.setValueAtTime(0.7,t+0.03);
g.gain.exponentialRampToValueAtTime(0.001,t+0.22);
n.connect(bp); bp.connect(g); g.connect(master);
n.start(t); n.stop(t+0.25); track(n);
```

Careful with exponentialRampToValueAtTime to 0.05 then setValueAtTime 0.8 — exponential ramp ends at 0.05 at t+0.01, then at t+0.012 set to 0.8 (jump). Fine. Ensure values > 0 always for exponential. 0.05 ok.

Scheduler with generation token:

```js
let playing=false, step=0, nextTime=0, timer=null;
const LOOKAHEAD=25, AHEAD=0.12;
function stepDuration(){ return 60/bpm/4; }
function scheduler(){
  if(!playing) return;
  while(nextTime < ctx.currentTime + AHEAD){
    const s=step;
    TRACKS.forEach((tr,i)=>{ if(state[i][s]) tr.synth(nextTime); });
    drawQueue.push({step:s, time:nextTime});
    nextTime += stepDuration();
    step=(step+1)%STEPS;
  }
}
function start(){
  ensureAudio();
  if(playing) return;
  playing=true;
  step=0;
  nextTime=ctx.currentTime+0.06;
  drawQueue.length=0;
  scheduler();
  timer=setInterval(scheduler, LOOKAHEAD);
  setPlayBtn...
 requestAnimationFrame(draw);
}
function stop(){
  playing=false;
  clearInterval(timer); timer=null;
  killAll();
  drawQueue.length=0;
  step=0;
  clearHighlight();
}
```

draw():
```js
function draw(){
  if(!playing) return;
  const t=ctx.currentTime;
  while(drawQueue.length && drawQueue[0].time<=t){
    last = drawQueue.shift();
    setPlayhead(last.step);
  }
  raf=requestAnimationFrame(draw);
}
```
Need to also cancel raf on stop. Keep a rafId.

BPM: range input 60-180 + number readout. Changing while playing just changes stepDuration for future notes — no duplicate sequences.

Also allow keyboard shortcut space to toggle play/stop? Nice but careful with focus on range. I'll add it but prevent default only when target isn't input/button... Actually space on a focused button triggers click. I'll skip global space handler to avoid conflicts, or handle it checking e.target.tagName not INPUT. Let's include: if code==='Space' && target is body → toggle. Fine.

Step click: ensureAudio on first gesture (so that context unlocked before Play too), toggle state, update DOM aria-pressed/class.

Also update display of pattern count? Not needed.

Status/hint: "Audio locked — click anything / press Play" small pill; hide once ctx running.

Also show current BPM in ms? Show "1 bar = 4 beats" note.

Also highlight beat numbers row1..16 with accents on 1,5,9,13.

Let me write the file now.

Also for the visual playhead, add class 'current' to steps of current column, and mark header markers.

Write it.

Edge: exponentialRampToValueAtTime requires time > previous event time. In clap, at t+0.024 I setValueAtTime(0.8) then ramp to 0.05 at t+0.034 — ok. Then setValueAtTime(0.7, t+0.03) — that's after t+0.034? No,0.03 < 0.034, which would be earlier than the ramp end — automation timeline out of order: setValueAtTime at a time earlier than the end of a running ramp... Actually the third burst ramp ends at t+0.034, and setValueAtTime(t+0.03) is before that. Web Audio spec: events are processed in time order; setValueAtTime inserts into the timeline sorted by time. If a scheduled event has time earlier than the current playback time when reached... It's fine because all events are in the future when scheduled, they get sorted. Hmm, but ramp ending at 0.034 and setValue at 0.03: order sorted: ... setValueAtTime(0.8 @ 0.024), setValueAtTime(0.7 @0.03), exponentialRamp ending at 0.034 (its start event is the last event before it, which would be the setValue at 0.03 → so ramp goes0.7 → 0.05 over 0.03-0.034, very fast). That works, ok but let me make it cleaner: use offsets [0, 0.011, 0.022] with ramps to 0.06 ending at off+0.01, then final setValueAtTime(0.6, t+0.033) then exponential to 0.001 at t+0.25. Clean ordering.

Let me write it.

Also add slight randomization for hats? Not required. Maybe add tiny velocity variation for hats on odd steps (quieter). Nice touch: hats on offbeat 16ths quieter — but state only has hats on every other step. I'll add a subtle velocity for step index odd → 0.6. Implement in schedule: pass velocity based on step parity for hats. Simple: in the synth call pass `s` and let hat use `const vel = (s%2===0)?0.55:0.35;`.

Now write.

Grid CSS details:

```css
.row{display:flex;align-items:center;gap:12px;}
.row-label{width:88px;flex:0 0 auto;font-weight:700;...}
.row-steps{display:flex;flex:1;gap:6px;}
.step{flex:1 1 0;min-width:0;aspect-ratio:1/1;border-radius:8px;...}
.step:nth-child(4n){margin-right:12px;}
.step:last-child{margin-right:0}
```

Total width: 16 steps +3 gaps of extra 12. On a 1200px screen: label 88 + gaps... fine. Add min-width to steps container via wrapper with overflow-x auto and min-width 640px.

Header markers row: label spacer + 16 markers matching the step widths. I'll instead put the beat number inside a marker row using same flex structure.

Let me write the whole thing.

For step colors per track: use CSS var --c set per row.

.step { background: color-mix? } avoid color-mix for compat? It's widely supported now (2023+). Safer: use opacity on a filled pseudo-element, or use background rgba white and border, with active state using var(--c). I'll do: inactive: background rgba(255,255,255,0.06); border1px rgba(255,255,255,0.12). Active: background var(--c); border-color var(--c); box-shadow glow. That avoids color-mix.

.step.current (playhead): outline: 2px solid #fff / brightness filter, and background for inactive current: rgba(255,255,255,0.22).

Add transition.

Write the file.

I noticed two typos I introduced in CSS: `background:#195madd;background:#1a5c80` and `color:#5d6removed;color:#5d6880`. Those are invalid then overridden by the following valid declaration, so they'd still render fine, but let me clean them up.

Now let's verify in the browser. Check console errors, initial rendering, toggle a step, play, watch playhead advance, change BPM, stop/start.

Note: in the sandboxed browser, Web Audio may or may not work (headless Chrome usually supports AudioContext). Let's check.

Now let me run it in the browser. I need to find how the browser tool works — the browser tool takes actions like "reload", "inspect", "screenshot", "click", etc. It presumably serves the cwd's index.html. Let me reload and inspect.

The JS parses. Now let's verify in the browser. I need to open index.html. The browser tool probably loads a local file. Let me try reload/screenshot.

Initial render looks great: default pattern shows kick on 1,5,9,13; snare on 5,13; hats on 8ths; clap on 13. Playhead at step 1 (first column highlighted). Status shows Stopped / Audio locked.

Now toggle a step (e.g., kick step 3, index 2 → row 1, col 3). Then click Play and check playhead advances and no console errors. Let's inspect audio context state too.

Let me toggle a step: kick row,3rd pad. Coordinates: kick row y≈246, steps x: 225, 281, 337, 393,... So step 3 (index 2) is x=337, y=246.

Actually to make the test more interesting, toggle snare step 1 (index 0) → x=225, y=304.

Let me first toggle and verify class change, then Play.

Let me toggle a step. Snare row is at y=304, first step x=225. Kick step 3 at x=337, y=246.

Hmm — I clicked on kick step 3 (x=337) and expected it to toggle ON, but the screenshot shows kick row still only steps 1,5,9,13 lit. The click may have landed but the pattern appears unchanged. Wait, maybe the click registered on step 3 but toggled... Let me check: kick row y=246, step3 x=337. In the image, x=337 in kick row is dark. So the toggle didn't happen, OR the click toggled it on and then off (two events?). Actually the click tool may dispatch a single click. Hmm.

Note the audio pill now says "AUDIO READY" — so the pointerdown handler fired and unlocked audio. So the gesture was received by the window. But the step didn't toggle? Maybe my coordinate hit a different element... Actually looking closer at the kick row: pads at x≈225(kick on), 281(off), 337(off), 393(off), 457(on)... The click at 337 should be step index 2 (off → on).

Maybe the screenshot was taken before the class applied? No, it's synchronous.

Wait — maybe the click actually landed at a y that's between rows, or the tool clicked and the state toggled twice (pointerdown + click?). My toggle handler is on "click" only. Hmm.

Let me inspect the DOM to check aria-pressed states.

Let me use the browser inspect action to check aria-pressed. The inspect action dumps DOM. But I need specific info. Let me try the browser tool with action "inspect" — it returns dom text. Better: use "type"? No. Maybe I can add a temporary probe... Actually the browser tool's "inspect" gives DOM text only, no custom JS evaluation.

Hmm, let me just try clicking again and take a screenshot to see if it toggles.

Now kick step 16 (last) is lit and step 2 shows hover. So my click coordinates are off: x=337,y=246 apparently hit... wait, kick step 16 is now ON. That means the first click (or second) hit the last kick pad at x≈1081. That's weird.

Oh! I see — the browser tool's click coordinates may be scaled differently. The screenshot is 1200×800 but the actual click coordinates might map to a different viewport scale. The instructions said "Coordinates use a 1200×800 viewport." The rendered image is 1200×800. Hmm.

But wait — the second click: kick step 3 didn't toggle; kick step 16 got toggled on, and there's a hover highlight at step 2 (x=281). That suggests the click event coordinates are being mapped weirdly, or perhaps my first click actually toggled step 16 on, and the second click did nothing visible? Actually now the hover state shows on kick step 2 (lighter), which is at x≈281,y≈246 — not where the mouse is supposedly (337,246). So there's an x-offset of about -56px between the tool's coordinates and the page.

Hmm, actually the previous screenshot (after first click) showed NO change and no hover. Maybe the click maps with some offset like page coordinates vs viewport.

Let me reconsider: maybe clicks are dispatched at the coordinates given, but the page inside the iframe is offset. The tool probably handles that.

Alternatively, the first click toggled kick step 16 on (looks like step 16 lit now). Wait, was step 16 lit in the first screenshot? Looking at first screenshot: kick row last pad (x≈1081) is dark. Second screenshot: lit. So the SECOND click toggled step 16 on. And the hover highlight appears at x≈281 which is 56px left of 337.

So the actual click position is ~56px to the left of what I specified: 337-56 = 281 → hover on step 2... but it toggled step 16?? That's inconsistent — hover at 281 but toggle at 1081.

Hmm, unless: the click at "337" landed at 281 (kick step 2), toggled it ON, and... no, step 2 shows hover (light) but not "on" (orange). Actually the light-gray fill at step 2 could be :hover. And step 16 orange is "on".

Wait — maybe the first click toggled step 3? Let me look at first screenshot again: kick steps lit: 1, 5, 9, 13. Step 16 dark. Second screenshot: 1,5, 9, 13, 16 lit, step 2 hover.

So one toggle happened between the two screenshots, at step 16. Strange.

Maybe the click is dispatched with an offset due to the device pixel ratio or the iframe scale, and hit-testing gives weird results. Let me try using `type` or check whether the browser tool has a different coordinate mapping. Better approach: use keyboard/DOM-driven verification instead of raw coordinates — but the tool doesn't offer arbitrary JS evaluation... Actually, maybe "inspect" runs some JS. Let me check what the browser tool supports: reload, inspect, screenshot, motion, click, drag, key, type, scroll.

Alternative approach: make the app's hit-testing irrelevant by clicking via a more reliable method — e.g., focus a step button with keyboard Tab and press Enter/Space. But buttons respond to Space/Enter natively via click. However my global keydown handler intercepts Space when tag === "BUTTON"? No — I excluded BUTTON and INPUT, so Space on a focused button will trigger the native click. Good.

But to focus a specific step, tabbing is tedious.

Hmm, maybe the coordinate mapping issue: The tool said "Coordinates use a 1200×800 viewport". The screenshot is exactly 1200×800. So click at (337,246) should be kick step 3.

Unless... the click landed on kick step 3 but the button element is at a different place than I think. Let me measure: panel starts x≈78, row-label width 92 + gap 12 → steps container starts at ≈ 78+16(padding)+92+12 = 198. Steps area width: 1200-78-78-16-16 = ~1012 minus label 92 minus gap 12 = ~908. 16 steps + 3 extra gaps of 12 = 908 → each step = (908-18-36)/16... let me compute: gaps: 15 gaps × 6px = 90, plus 3×12 extra = 36 → 126 total gaps. Steps total = 908-126 = 782 → each48.9px. So step centers: start 198, step i center = 198 + 6 + i*(48.9+6) + 24.4 ≈ 228.4 + 54.9i. Step 0: 228, step1: 283, step2: 338, step3: 393... That matches what I estimated from the image (225, 281, 337, 393). So (337,246) = kick step 2 (0-indexed). It should have toggled kick step 2 ON. But instead step 16 toggled.

Something's off with the click dispatch. Possibly the click is dispatched at document coordinates rather than viewport, and there's a scroll offset... but the page isn't scrolled.

Or perhaps the tool clicked twice (two frames) — first click toggled something, second click toggled back? No, only one change.

Let me try a different approach: use `key` action to Tab into the grid and press Enter, or better — let me check if there's an offset by clicking the Play button at its known location and seeing if it starts.

Actually the safest approach: add temporary debugging is not ideal. Let me just try clicking at slightly different coordinates and observe. Let me click Play at (141,128) and see if playback starts.

Playback started. The Play button at (141,128) worked fine — that's a large target. The button now reads "Playing", pill "PLAYING · BAR 1", audio ready. The current step highlight is on column 1 (white outline on kick/hat step 1). 

Note: kick step 16 is no longer lit in this screenshot! Interesting — the "on" kick at step 16 disappeared. Hmm, that's odd. Wait, look: kick row now shows 1,5,9,13. Step 16 dark again. So the earlier "step 16 on" might have been a rendering artifact or the click toggled it and then... Actually maybe the earlier click at (337,246) dispatched at a scaled position hitting kick step 16, and then a subsequent mouseover/pointer... no, it should persist.

Hmm, wait. Maybe the previous screenshot's step-16 highlight was actually the *hover* state on step 16 (light gray-orange). Looking at that screenshot: step 16 fill was a muted orange/tan (#d08?), lighter than the "on" orange of step 13. And the step-2 lighter fill was hover. So possibly two hover artifacts... but hovers don't render in a headless screenshot unless a real mouse moved.

Whatever the cause, the reliable path: clicks near the edges may be mapped oddly. Let me now use motion frames to verify playhead advance. Let me capture a few motion frames while playing.

Actually, the key verification: playhead advances across grid. Let me take screenshots rapidly. The `motion` action captures frames over time. Let me use it.

Let me check the playhead advance by taking several screenshots in quick succession. The `motion` action might do that. Let me try it.

The playhead advances: frames show column 5 (beat 2), column 9 (beat 3), column 14 (beat 4), column 2. Great — the playhead sweeps the grid and wraps around. Also, the Play button is now green with "Playing" and the status pill pulses.

Also note: kick step 16 is no longer lit — so the earlier apparent toggle was... hmm, it now shows kick 1,5,9,13 only. So that second click's effect is gone. Wait, but the pattern is state in memory; it wouldn't revert. Unless the earlier click toggled step 16 ON and the earlier screenshot showed it ON, and now it's OFF again — nothing would have turned it off.

Hmm wait. Look at frame 3 above: kick row shows 1, 5, 9, 13 lit and column 14 (index 13) is the playhead. Kick step 16 (last) dark. So state has step 16 off. So that earlier "on" at step 16 must have been a hover style? No, hover is lighter gray, that was orange.

Hmm, actually in the second screenshot, step 16 of kick had a muted orange color — that could be :hover on... no, hover is `background:rgba(255,255,255,.13)` which is gray.

Possibility: the click dispatched at (337,246) actually landed on kick step 16 (toggle ON) via a mousedown/mouseup at different coordinates producing a click on the common ancestor? No — common ancestor would be the grid container, not another button.

Alternatively the browser harness sends a click and then moves the mouse to (0,0) or dispatches extra events. Or perhaps the screenshot with step-16 orange was taken mid-transition.

Whatever happened, current behavior looks right. But I want to verify pad toggling deterministically. The coordinate mapping seems unreliable for small targets — maybe the harness clicks with some offset/scaling. Earlier, my click at (337,246) produced hover at x≈281 (step 2) and an ON at step 16. That's inconsistent, suggesting the harness might dispatch mousedown at one point and mouseup/click elsewhere, or the page had scrolled slightly.

Wait — actually! I bet the harness scrolls the page. Hmm, no.

Let me look again at second screenshot: step 2 of kick appeared light gray (hover) and step 16 orange (on). If the actual click coordinates were scaled: maybe the iframe viewport is smaller than 1200×800 (e.g., 1140×760) and coordinates map proportionally... If actual x = 337 * (something)... To land on step 16 (x≈1081) from a requested 337, the scale factor would be ~3.2. No.

Alternatively, the click at (337,246) toggled kick step 2 ON, and then... the reload? No reload happened between.

OK here's another thought: maybe the click toggled step 2 ON, and then the harness moved the mouse away and my screenshot showed... no, step 2 was gray not orange in screenshot 2, and step 16 was orange.

Hmm, but wait — maybe I misread which screenshot. In screenshot 2 (after second click), kick steps: 1 (orange), 2 (light gray), 5(orange), 9, 13 (orange), 16 (muted orange/tan). The tan at 16 — the `:hover` style is rgba(255,255,255,.13) over the dark panel = grayish. The16 pad looked distinctly orange-tan. Could be a CSS transition mid-frame? The `.step` has `transition: background .08s linear`. If a screenshot is captured immediately after a click, a newly-activated pad may be caught mid-transition (partially orange). That explains the muted orange at 16! But why step 16 and not step 2?

Unless the two clicks hit different pads due to the harness's mouse handling: first click → no visible change (maybe it hit a gap), second click → step 16? Still weird mapping.

Let me stop speculating and test toggling deterministically by clicking on a pad with a large margin, e.g., snare step 1 at approximately (225, 304). If offsets are inconsistent, I'll instead verify toggling via keyboard (Tab + Enter) or by temporarily exposing state to the DOM.

Actually, a cleaner approach: add a `data-step`/`data-track` attribute (already have aria-label) and also expose the sequencer state on `window.__seq` for testing? That's test-only pollution, but a small debug handle is acceptable... I'd rather not.

Better: use the `key` action to press Tab to move focus through buttons and Enter to activate. But I can't see focus in DOM.

Alternative: the harness's "click" may dispatch a real CDP Input.dispatchMouseEvent at those coords — that should be accurate. Let me test by clicking exactly on a pad center and then screenshotting after a moment (so transitions settle). Let me click snare step 1: x≈225, y≈304.

Actually — OH. I think I understand now! In screenshot 1 (after first click), the DOM dump showed nothing about it, but look at screenshot 1 again: kick row pads: 1 orange, 2 gray-dark, 3 gray-dark... Actually in screenshot 1, kick step 2 (x≈281) appears slightly lighter (hover) — that was the first click's hover. And step 3 at 337 appeared dark.

Hmm, in screenshot 1 kick step 16 appeared dark. In screenshot 2, kick step 16 orange + step 2 hover-light.

Theory: the harness click dispatches mousedown+mouseup at slightly different positions (a "drag"-like quirk) OR the page is inside an iframe whose coordinates the harness maps using the iframe's bounding box while I give coordinates in the outer viewport... If the iframe is offset within the page, then requested (337,246) → actual (337 - dx, 246 - dy). To go from 337 → 1081 is impossible with a negative offset.

Wait... maybe I have it backwards: maybe the click DID toggle kick step 2 ON (at 337) but I misidentified the pads. Let me recount the pads in screenshot 2. Pad centers for kick: 225 (on), 281 (light/hover), 337 (dark), 393 (dark), 457 (on), 513, 569, 625 (dark), 687 (on), 743, 799, 855 (dark), 917 (on), 973, 1029 (dark), 1081 (tan/on).

Hmm, so in screenshot 2 the highlighted-at-281 is "step 2" (index 1) which appears light gray = hover, and 1081 = step 16 index 15 shows muted orange.

You know what — maybe the harness dispatched TWO clicks total at slightly different coordinates: first at (337,246) → toggled index 2 ON; second at (337,246) → toggled index 2 OFF. Net zero on step 2. And step 16's orange was... nothing.

But then what turned step 16 orange in screenshot 2 and dark by the time Play was pressed? Nothing toggles it off except a click. Unless the harness's click sequence includes a mousedown at target and mouseup elsewhere — producing a click on a common ancestor (no toggle) — plus the visible "hover" artifacts.

I'm going in circles. The important question: does clicking a pad toggle it? Let me test cleanly now: click once on snare step 1 (x≈225,y≈304), wait, screenshot. If snare step 1 turns pink → toggling works, and the earlier confusion was a harness quirk.

The tool output format seems to be: my "thinking" text is interleaved with tool results — the harness seems to be running tools in a different order than I intend. Interesting: my bash commands are trivial echoes, so they're harmless. I should just call the browser tool directly.

Let me click the snare step 1 pad.

Snare step 1 is still off; nothing toggled. The playhead is on column 6 (index 5). The click at (225,304) apparently did nothing visible.

Hmm — but the click at Play worked (141,128). Maybe the harness click coordinates are correct, but there's something about clicking pads: my handler is attached to the button's "click" event. It should fire.

Wait... maybe the issue: `.steps .step:nth-child(4n){margin-right:12px}` — that's fine.

Hold on. Look at this screenshot: playhead at column index 5 (x≈567). Snare row: is there a hover highlight anywhere? No.

Is it possible that the click actually toggled snare step 1 ON and then OFF (double dispatch)? That would explain: click → two click events → net zero. And for the earlier kick case, first click may have double-toggled (on+off = no change) and the second click left it ON... but we observed the opposite ordering.

Let me test the double-dispatch theory: the harness may dispatch both a synthesized mouse click AND the resulting DOM click... no.

Better test: click a pad and immediately inspect the aria-pressed attributes. The `inspect` action dumps DOM — does it include attributes? It dumped text content only. Hmm.

Let me add a visible side-channel: make the status pill briefly flash "Step X toggled on/off" when a pad is toggled. That's actually a nice UI affordance (feedback) AND gives me a deterministic way to count toggles. Even better: keep a persistent "active steps: N" counter in the status bar. Then I can count toggles via the DOM text, which the tool reports.

I'll add a small readout "PATTERN: N STEPS" that updates on toggle/clear. This is genuinely useful UI and makes verification deterministic.

Also, to make clicking more forgiving, that's not the issue.

Let me also consider: maybe clicks on pads ARE working, but my earlier clicks hit the wrong pads due to coordinate scaling, and this last click at (225,304) hit the snare row's step 1... and toggled it ON, then the screenshot was taken... it shows off. So no.

Unless the toggle happened on a DIFFERENT row/step that I overlooked. In this screenshot, everything matches the default pattern. Nothing changed.

Adding the counter will reveal the truth. Let me implement:
- In the status area, add a pill: "PATTERN ·13 ON" (count of active steps).
- Update it in toggle handler and clearAll.

Also add a transient message in stateText? Keep it simple: just the counter.

Let me edit the file.

Note: the default pattern has kick 4 + snare 2 + hats 8 + clap 1 = 15 steps on. Wait: kick [0,4,8,12] = 4, snare [4,12] = 2, hats 8, clap [12] = 1 → total 15. Earlier the DOM said "13 steps on"? No, I wrote "13 steps on" as static HTML placeholder, which will be replaced on init. Fine, but let me make the placeholder text neutral ("— steps on") to avoid confusion. Actually it gets set immediately by updatePatternCount(). It'll show "15 steps on". Let me just reload and check.

Now reload and test clicking a pad with the counter as ground truth.

"15 STEPS ON" confirms the counter works. Now click a pad and check whether the count changes to 16.

Let me click the snare step 1 (x=225, y=304).

The count went 15 → 16. The click DID work: snare step 16 (last column) is now lit (mid-transition pink at x≈1081). So the harness's click coordinates map to a *different* pad than the visual center I picked — there's a horizontal offset in this sandbox (clicks land ~56px to the right? No: I asked for x=225 (step 1) and it toggled step 16...).

Wait: I asked for (225,304) = snare step 1, and it toggled the LAST snare pad (x≈1081). So the click landed far to the RIGHT of the requested point. That's a +856px shift?! That's bizarre.

Hmm, unless... there's horizontal scroll: the `.scroll` container has overflow-x auto with min-width 660px on .grid. If the browser window is narrower than 1200 in the harness... but the screenshot is 1200 wide.

OH WAIT. I think I finally get it. The harness might be dispatching the click inside an iframe that is *scaled*: the iframe's internal viewport might be e.g. 1200×800 CSS px but the outer page is smaller/larger, and coordinates get transformed non-uniformly.

Requested x=225 → actual x≈1081. Earlier: requested 337 → actual 1081 too?? Both landed on the last column. And the earlier "hover at 281" observation suggests the pointer ended up somewhere else entirely.

Hmm, if clicks always land on the last column regardless of x, that means the hit test uses a fixed point... Both clicks landing on step 16 (the rightmost pad) is suspicious: maybe the click is dispatched at the *right edge* of the element/container.

Actually here's a plausible explanation: the harness may dispatch a "click" by synthesizing mouse events at coordinates relative to the *iframe viewport in CSS pixels*, but the page inside is rendered at a different scale (e.g., devicePixelRatio 2 with a 2400px-wide layout shown as 1200). If the layout is 2400 wide and my coordinate 225 is interpreted in the 1200-wide outer space, then the inner coordinate would be 225*2 = 450... still not 1081.

Alternatively the inner layout is ~590px wide (a mobile-ish viewport) and clicks map 225 → right edge...Whatever the cause, the good news: pads DO toggle correctly (count 16, pad lit, mid-transition color). The coordinate mapping is a harness quirk, not an app bug. The earlier confusion is resolved: my clicks were landing on different pads than I targeted (rightmost column), and since clicking a pad twice toggles it on/off, the "no change" observations were actually a toggle-on followed by toggle-off (two clicks on the same rightmost pad — which explains step 16 of kick turning on and then off again!).

That's consistent: kick step 16 turned ON after click2, then click 3 (on Play) didn't affect it... hmm, but by the time of the motion frames, kick 16 was off. Well, one of the clicks toggled it back off. Close enough — the pattern of evidence now is consistent with a fixed hit point near the right edge for pad clicks.

Hmm, but hold on: is it a fixed hit point, or is it that clicks land where I ask but with a large offset? For the Play button I asked (141,128) and it played. If there were a huge rightward offset, the Play click would have hit something else. So the offset theory fails for the transport row.

New theory: the click is dispatched at the requested coordinates, but the *hit-tested element* is determined after the page re-renders/scrolls. Hmm.

Alternative theory: the `.scroll` container scrolled horizontally between screenshots? If `.scroll` scrolled right by ~856px, then a click at x=225 would land on a pad that visually appears at x=1081 in the unscrolled layout! And the Play button (outside .scroll) would be unaffected. THAT FITS PERFECTLY.

Why would .scroll be scrolled right? Because clicking a button focuses it, and the browser scrolls the focused element into view horizontally within the scroll container! The last pad in my earlier clicks... no wait, the first click targeted step 2 (well within view).

Hmm, but the scroll container's content is min-width 660px within a ~1090px available width, so there's nothing to scroll — unless the harness's actual viewport is narrower than 1200 CSS px and the screenshots are upscaled. That's very plausible: the browser viewport might be, say, 600×400 CSS px, screenshots upscaled to 1200×800 for me, and my coordinates (1200-space) get divided by 2 → x=225 → 112 CSS px... that lands on the Play button, not the last pad.

OK, let me test the scroll hypothesis directly: click a pad in the middle and take a screenshot to see if the grid has scrolled. In the latest screenshot, the grid is NOT scrolled (beat numbers1-4 all visible, panel aligned). So no scroll.

Alright, let me approach differently and stop reverse-engineering the harness. The functional evidence is what matters:
- Pads toggle (count 15→16, pad lit) ✓
- Playhead advances ✓
- Audio unlocks ✓

Remaining to verify: BPM response, Stop resets playhead & cancels, pattern retention, Clear.

To test these without relying on precise coordinates, I can:
1. Set BPM by clicking on the slider (large target) — but again coordinates are suspect. Better: use keyboard! Focus the range input and press ArrowLeft/ArrowRight (my handler updates bpm). The `key` action can send keys. To focus the slider I can click it (it's a big 210px target) then press ArrowRight several times. If clicks land somewhere weird, I can instead Tab from body.

Actually, I realize the safest interpretation: clicks may be landing at the right edge of the row. If I click the *slider*, a right-edge landing is still on the slider (it ends at x≈908... no, the tempo box spans735-1108; right edge lands on the "120 BPM" readout, not the slider).

Hmm. Let me test with the `type` action? The range input doesn't accept typing.

Alternative: since the keyboard handler works on INPUT focus, I could focus the slider by clicking it, then verify focus took by pressing ArrowRight and reading the BPM value in the DOM dump. If the click didn't focus the input, the keypress would do nothing (my handler checks tag === INPUT) and the value would stay 120. Let's try: click around x=850,y=128 (on the slider track), then key ArrowRight.

Note: clicking a range input at a position also SETS the value to that position — which itself is a BPM change test. If the click lands at the right edge of the tempo box (x≈1108), that's outside the input, so no set.

Let me try: click slider track at x=850,y=128, then screenshot to read the BPM value.

Actually, better idea to sidestep all coordinate weirdness for the remaining checks: use keyboard only.
- Space toggles play/stop (works when focus is not on a BUTTON or INPUT).
- Arrow keys change BPM when the range input is focused.

To focus the range input without a click, I can Tab. Starting from body, Tab order: Play → Stop → Clear → range input → 64 step buttons. So 4 Tabs from the body (after a reload, focus is on body). The `key` action might support "Tab" repeats — I can send Tab 4 times, then ArrowRight.

Hmm, but if any earlier click left focus on a pad, Tab ordering shifts. Let me reload first (resets focus to body), then send Tab×4, then ArrowRight×3, then screenshot: BPM should read 123 and the slider fill should move.

Let me do that. First reload.

Pattern reset to default (15 steps on) after reload. Good — confirms the default pattern loads fresh.

Now Tab ×4 to focus the range input, then ArrowRight ×3. Let me send keys. The `key` action takes a `text`? Probably `key` with `text` = key name. Let me try `{"action":"key","text":"Tab"}`. I'll do one at a time to be safe. Actually I can send multiple via key with text like "Tab Tab"? Unclear. Let me try one Tab and see if focus appears (screenshot shows focus ring).

Let me send4 Tabs individually.

Let me send the keys. I'll use the key action with text "Tab".

Tab focused the Play button (visible focus ring around Play). Keyboard focus works and audio unlocked (the keydown gesture). Now Tab 3 more times: Stop, Clear, then the range input. Then ArrowRight.

Let me send Tab three times in a single call if possible — the tool takes one `text`. I'll send them sequentially but quickly. Actually maybe `text` can be "Tab Tab Tab"? Unclear; risky. Send individually.

Alternatively, from Play (focused), I can press Tab 3 times → Stop → Clear → slider. Let me do3 more Tab calls then ArrowRight, then screenshot.

The tool output format is odd — my "thinking" narration is being echoed as tool results. I need to be careful: the harness seems to run my bash commands. Let me just make the browser calls.

Note: the sequence "tab x4 then arrowright x3" comment is mine. Let me send Tab three times.

Focus moved to Stop. Two more Tabs → Clear → slider. Then ArrowRight ×3.

Time is limited. Let me be efficient: Tab once more to reach the slider, then ArrowRight ×3, verify BPM changes to 123, then Space to start, verify playhead, Space to stop, verify reset, then Clear (click may be unreliable — but I can focus Clear with Tab and press Enter).

Let me batch: key Tab (to slider), then key ArrowRight, then reload-free screenshot. I can only do one action per tool call, but I can issue multiple tool calls in one block? The instructions say independent calls in the same block. Let me do Tab and then... they're dependent (sequential), so I must do them one at a time. Time budget: ~5 min. Let me prioritize:

1. Tab (reach slider) + ArrowRight ×2 → BPM change verified (2 calls)
2. Space → start; motion/screenshot → playhead + BPM faster (2 calls)
3. Space → stop; screenshot → reset (2 calls)
4. Tab to Clear + Enter → clear verified (2 calls)

That's ~8 calls. Feasible if quick. Let me combine where possible: after starting playback I can change BPM while playing.

Go.

Focus is now on the range input (focus ring around the whole tempo box). Now press ArrowRight 3 times → 123 BPM. Let me press ArrowRight and check.

BPM went 120 → 122 (slider default step is 2? No, step=1... but two increments happened? Possibly the harness dispatched the key twice, or my keydown handler plus native range handling both fired: native range ArrowRight increments by 1 → 121, and my handler setBpm(bpm+1) using the *variable* which was 120 → 121... that gives 121, not 122. Hmm, unless both ran: native input event fired → my input listener setBpm(121) → bpm=121; then my keydown handler (fires before input event? keydown fires first) setBpm(120+1)=121... Order: keydown → native default (value changes, input event) → my input listener. My keydown handler runs first: setBpm(121) sets value=121. Then native default applies ArrowRight to the *current* value (121) → 122, input event → setBpm(122). Net: +2 per press. Minor: doubling. That's a small bug — arrow keys on the focused slider move BPM by 2 instead of 1.

Fix: in the keydown handler, don't call setBpm for the range input's arrows — the native slider already handles them (and my input listener syncs the readout). Remove the ArrowLeft/ArrowRight branches for INPUT. But then the footer says ←/→ tempo — still true natively. 

Let me make that essential fix quickly. Then test BPM response during playback and stop/reset.

Actually time is tight (~4 min). This is a genuine (if minor) bug worth fixing: remove the arrow-key branch.

Now reload, verify: slider arrows single-step, start playback with Space (focus must not be on button/input — after reload, focus on body), verify playhead, change BPM during playback, stop with Space, verify playhead resets to step 1, and Clear.

Let me be quick. Reload, then Tab×4 to slider, ArrowLeft (→119), then Space? Space with focus on INPUT is excluded from my handler, so playback won't start. I'd need to blur. Simpler: after verifying the slider, click Play (large target, works), then test BPM during playback via slider focus + arrow (works while playing since handler is on input event), then Space to stop requires focus on body — clicking a pad would blur... clicking Play button again toggles stop. Since Play button click worked before at (141,128), I can use it to stop.

Plan (4-5 calls):
1. reload
2. key Tab ×4 (4 calls) — too many. Alternative: click Play directly (1 call) to start; then key Tab is relative to current focus (Play button focused after click? clicking a button focuses it) → Tab ×3 to slider → ArrowLeft → check BPM while playing.
3. Then click Play again → stop → screenshot: playhead on step 1, "STOPPED".
4. Then Tab to Clear + Enter, or click Clear.

That's about 8 calls; ~3.5 min. Tight but doable if I stop narrating with bash. Let me move fast.

Actually to save calls: skip re-verifying the slider single-step (the fix is obviously correct). Instead:
1. reload
2. click Play (141,128) → playing
3. motion frames → playhead advancing4. click Play again → stop → screenshot → verify playhead reset & pattern retained
5. Tab×3 + Enter on Clear → verify0 steps

Let me go. To minimize, I'll combine the BPM check: while playing, focus slider via Tab×3 from Play and press ArrowLeft, then motion. That's more calls. Given time, I'll do the essentials: play/stop/reset + clear. The BPM response was already verified (120→122 readout + slider fill moved), and the scheduler reads bpm each step — sound timing change is not observable in screenshots anyway.

Fresh reload: 120 BPM, default pattern, stopped, playhead on step 1. Now start playback and verify the whole play→stop→restart cycle. Click Play at (141,128).

Playing, playhead on step 1. Now let it run a moment, then click Stop and verify the playhead resets to step 1 and pattern retained. Let me capture motion frames first to confirm advance (already verified, skip to save time). Click Stop at (267,128).

Actually, I want to verify: after a few steps of playback, Stop → playhead back at step 1. Let me wait via a motion capture (which advances ~1.6s) and then stop. To save calls, just click Stop now-ish — but then the playhead would be near step 1 anyway, a weak test. Let me do one motion call (~1.6s → ~5 steps at 120bpm) then click Stop.

Playhead advances: step 15 (index 14) → step 3 (index 2, wrapped past the end) → step 7, step 12. Confirms wrap-around and continuous advancement. Now click Stop (267,128) and verify the playhead resets to step 1 and the pattern is retained.

Stop worked: status "STOPPED", button back to "Play", pattern retained (15 steps on), and the playhead highlight column is column index 14 (x≈973 — outlined pads in that column). Wait — that's the *previous* playhead position, not reset to step 1!

Look: pads at x≈973 (column 15, index 14) have white outlines in all rows. So the playhead is stuck at index 14 instead of being cleared/reset to step 0.

Why? In stop() I call `setPlayhead(0)`. setPlayhead(0) with currentCol=14: it removes 'current' from column 14 for all rows, then adds 'current' to column 0. But the screenshot shows column 14 highlighted, not column 0.

Hmm wait. Look more carefully: the pads at x≈973 have a white outline (2px white ring). Column 0 (x≈225) has no ring — the kick pad there is just orange.

So the highlight is at column 14 after stop. That means `setPlayhead(0)` didn't run, or... Actually — order in stop(): 
```
playing=false;
clearInterval...
cancelAnimationFrame
killScheduled()
drawQueue.length=0
step=0
setPlayhead(0)
updateTransportUI()  // which calls clearPlayheadMarks()
```
updateTransportUI calls clearPlayheadMarks() when !playing, which removes 'current' from currentCol and restores beat label. But setPlayhead(0) already moved currentCol to 0 and set column 0 as current. Then clearPlayheadMarks removes current from column 0 (correct, since we're stopped) — so NO column should be highlighted. But column 14 IS highlighted.

Unless... setPlayhead(0) was called while currentCol === 14: it removes from 14 and adds to 0. Then clearPlayheadMarks removes from 0. Net: nothing highlighted. But the screenshot shows 14 highlighted.

So something re-added 'current' to column 14 after stop. What? The rAF draw loop! cancelAnimationFrame(rafId) — but the draw function may have already been re-queued: if `draw()` was executing concurrently... no, single-threaded.

AH WAIT. I see it: in `draw()`, `rafId = requestAnimationFrame(draw)` at the end. If stop() cancels the *last* rafId, fine. But consider: stop() is called from a click handler. Between the last draw() call and stop(), no new frame was scheduled except the one stored in rafId. So cancellation should work.

Hmm, but what if the click handler on the Stop button... Actually, the visible highlight at14 could be a leftover from the LAST draw before stop, and clearPlayheadMarks removed the class from currentCol — but if currentCol was already changed... 

Let me re-read setPlayhead:
```js
function setPlayhead(col){
  if (currentCol === col) return;
  if (currentCol >= 0){ remove from currentCol; fix beat labels }
  add 'current' to col; beatEls[col].add strong; currentCol = col;
}
```
and clearPlayheadMarks removes from currentCol (without resetting currentCol to -1!). So after stop: currentCol stays 0 with no highlight. Then when playback restarts, setPlayhead(0) returns early (currentCol===0) and never adds the highlight to column 0! That's a real bug: after stop→play, the first column wouldn't show as current until the playhead moves to column 1. (Earlier we saw the highlight on step 1 right after start, because at that time currentCol was -1 initially.)

But that doesn't explain the column-14 ring in this screenshot...

Unless the timing: the click on Stop happened, stop() ran, then the browser painted... the ring at 14. Hmm, if stop() ran but setPlayhead(0) didn't remove column 14's class because currentCol had been changed to something else... 

Oh! Here's the thing — `setPlayhead` is also called from draw() with the *last* queue entry. Suppose draw() ran and set currentCol=14 (adding class to 14). Then stop() runs: setPlayhead(0) → removes 14's class, adds 0's. Then updateTransportUI → clearPlayheadMarks → removes 0's class. Column 14 ring shouldn't exist.

Unless the click landed on a pad and... no.

Wait, maybe the ring at column 14 is a `:hover`/`:focus-visible` artifact? The mouse is at (267,128) — not on the grid. Hmm, but `.step.current` uses box-shadow 0 0 0 2px white. What else could produce a white 2px ring on4 pads in the same column? Only .step.current.

So how? Let me reconsider: maybe the rAF loop DID keep running. cancelAnimationFrame(rafId): rafId holds the id from the most recent schedule. But look at draw(): `rafId = requestAnimationFrame(draw)` — each frame overwrites rafId with the *new* id. If stop() runs *between* the callback firing and... it's synchronous, no gap.

BUT: `start()` also sets rafId. And notice — in draw(), when the queue is empty and time passes, nothing. Fine.

Hmm, what if `playing` was still true when the final draw ran? stop() sets playing=false first, so draw returns early.

Let me look at the screenshot once more... Actually, maybe I'm misreading: could those outlines be on column index 14 or15? x≈973 = step index (973-228)/55≈ 13.5 → column 14. Yes, 14.

Alternatively — the ring might be the result of `killScheduled()` and the highlight from... no.

Hold on! Maybe the click on "Stop" triggered `stop()` twice? No.

Hmm, what about this: the last motion frame showed the playhead at column 12 (x≈851). Between then and my click, ~1-2 seconds passed → columns 13, 14, 15, 0... The click's screenshot shows14 highlighted. If draw() kept running after stop() (bug), the highlight would have advanced further, not stopped at 14. Unless the rAF loop stopped exactly when column 14 was current — which is what cancelAnimationFrame does: it cancels the *pending* frame. If stop() happened while the pending frame was scheduled, the last painted column stays wherever it was —14! That's exactly consistent: the last column painted before cancel was 14, and clearPlayheadMarks failed to remove it.

Why would clearPlayheadMarks fail? Because `currentCol` !== 14 at that moment. Look: setPlayhead(14) set currentCol=14 and added the class. Then stop → setPlayhead(0): currentCol===14 → remove from 14, add to 0, currentCol=0. Then clearPlayheadMarks removes from 0. Column 14 should be clean...

UNLESS setPlayhead(0) returned early because currentCol was already 0! Could currentCol have been 0 while column 14 visually showed the ring? Yes — if the LAST draw() call set the playhead to 0 (wrapped around) but the paint... no, class changes paint together.

I'm burning time. Let me just fix it robustly and simply: rewrite the playhead code to be bulletproof:
- Keep a `currentCol` variable; in setPlayhead, always clear ALL columns first (querySelectorAll('.step.current') → remove), then apply to col. Cheap (64 elements).
- In stop(), call `resetPlayhead()` which clears all marks and sets currentCol=-1 (so the next start always paints column 0).
- Remove clearPlayheadMarks from updateTransportUI (call resetPlayhead explicitly in stop instead).

This removes the fragile state. Let me implement and re-verify.

Note: setPlayhead(0) is still called in start() — with the new implementation it always paints. Good.

Reload and verify: play → stop → playhead clears on stop, restarts from step 1. Time is very tight (~2 min). Let me do reload → click Play → click Stop → screenshot. That's 3 calls. Then done.

Fresh state, no stray highlight. Now play, let it run, then stop, and check the playhead resets cleanly. Play click at (141,128).

Playing with the playhead on step 1. Now let it run a bit and stop. I'll do one motion capture then click Stop. Time is nearly out — let me combine: motion (advances ~1.6s) then stop.

Playhead advances cleanly: column 10 → 15 → 3 → 7. Exactly one column highlighted at a time (the new clearAllMarks approach works). Now click Stop and verify no highlight remains and pattern retained.