D3 Examples - Selection bug

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://cdn.jsdelivr.net/npm/vega@5"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-lite@3"></script>
    <script src="https://cdn.jsdelivr.net/npm/vega-embed@4"></script>
</head>
​
<body>  
  <div id="vis"></div>
  
  <script>
    vegaEmbed('#vis', {
      mark: 'point',
      encoding: {
        x: {field: 'a', type: 'quantitative'},
        y: {field: 'b', type: 'quantitative'}
      },
      selection: {
        grid: {
          type: 'interval',
          bind: 'scales'
        }
      }
    })
    .then(result => {
        result.view.insert(
        'source',
        [{a: 1, b: 2}, {a: 3, b: 2}, {a: 3, b: 1}]
      )  
    })
      .catch(console.warn);
  </script>
</body>
    