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chromium/third_party/blink/web_tests/fast/canvas/canvas-no-alpha-invalidation.html 

<!DOCTYPE html>
<script src="../../resources/run-after-layout-and-paint.js"></script>
<p>Expect to see a black square below this line.</p>
<canvas id = 'c' width='100' height='100'></canvas>
<script>
if (window.testRunner) {
  testRunner.waitUntilDone();
  runAfterLayoutAndPaint(function(){
    // Verify that creating a context with alpha:false trigger a graphics
    // update even though nothing is drawn to the canvas.
    document.getElementById('c').getContext('2d', { alpha:false });
    testRunner.notifyDone();
  });
} else {
  console.log('ERROR: This test requires the testRunner interface.')
}
</script>