Spot the bug - #138: Neon Ring Canvas

Why is my glowing neon ring rendering stretched and distorted?

<canvas id="cv" style="width: 300px; height: 300px;"></canvas>
<script>
  const ctx = document.getElementById('cv').getContext('2d');
  ctx.arc(150, 150, 100, 0, Math.PI * 2);
  ctx.strokeStyle = '#00ffcc';
  ctx.lineWidth = 8;
  ctx.stroke();
</script>

Reply with what is broken and how you would fix it.