third_party/blink/perf_tests/paint/gap-decorations-grid.html

<!DOCTYPE html>
<script src="../resources/runner.js"></script>
<script src="resources/paint.js"></script>
<style>
  html, body {
    margin: 0;
    padding: 0;
  }

  #grid-container {
    display: grid;

    grid-auto-flow: row;
    grid-template-columns:
      repeat(3750, 20px 30px 15px 40px);

    width: 18000px;

    row-gap: 6px;
    column-gap: 6px;

    column-rule: 2px solid blue;
    row-rule: 2px solid red;

    column-rule-break: intersection;
    row-rule-break: intersection;

    column-rule-inset: 0;
    row-rule-inset: 2px;
  }

  .item {
    height: 10px;
    background-color: green;
  }
</style>

<div id="grid-container"></div>

<script>
  (function() {
    const container = document.getElementById('grid-container');

    function populateContainer() {
      const itemCount = 150000;
      const fragment = document.createDocumentFragment();

      for (let i = 0; i < itemCount; ++i) {
        const div = document.createElement('div');
        div.className = 'item';
        fragment.appendChild(div);
      }

      container.appendChild(fragment);
    }

    populateContainer();

    var wide = false;

    measurePaint({
      run: function() {
        container.style.width = wide ? "18000px" : "21000px";
        wide = !wide;
      }
    });
  })();
</script>