chromium/third_party/blink/web_tests/paint/invalidation/canvas-resize.html

<!DOCTYPE html>
<style>
canvas {
  position: absolute;
  width: 600px;
  height: 500px;
  top: 50px;
  left: 50px;
}
</style>
Tests paint invalidation of canvas when it's resized. Passes if there is a whole green circle.
<canvas id="canvas" width="500" height="500"></canvas>

<script src="resources/text-based-repaint.js"></script>
<script>
function repaintTest() {
  document.getElementById('canvas').style.width = '500px';
}
onload = runRepaintAndPixelTest;

var ctx = document.getElementById('canvas').getContext('2d');
ctx.beginPath();
ctx.arc(250, 250, 250, 0, 2 * Math.PI);
ctx.fillStyle = 'green';
ctx.fill();
</script>