third_party/WebKit/LayoutTests/fast/canvas/OffscreenCanvas-webgl-preserveDrawingBuffer.html

<script src="../../resources/testharness.js"></script>
<script src="../../resources/testharnessreport.js"></script>
<script>

function getPixelsFromOffscreenWebgl(preserveFlag) {
  var canvas = document.createElement("canvas");
  var offscreenCanvas = canvas.transferControlToOffscreen();
  var gl = offscreenCanvas.getContext("webgl", {preserveDrawingBuffer: preserveFlag});

  // Draw some color on gl and commit
  gl.clearColor(1, 0, 1, 1);
  gl.clear(gl.COLOR_BUFFER_BIT);
  gl.commit();

  var pixels = new Uint8Array(50 * 50 * 4);
  gl.readPixels(0, 0, 50, 50, gl.RGBA, gl.UNSIGNED_BYTE, pixels);
  return pixels;
}

test(function() {
  var pixelsPreserve = getPixelsFromOffscreenWebgl(true);
  assert_equals(pixelsPreserve[0], 255);
  assert_equals(pixelsPreserve[1], 0);
  assert_equals(pixelsPreserve[2], 255);
  assert_equals(pixelsPreserve[3], 255);
}, "test if OffscreenCanvas.webgl retains context if preserveDrawingBuffer is true.");

test(function() {
  var pixelsNoPreserve = getPixelsFromOffscreenWebgl(false);
  assert_equals(pixelsNoPreserve[0], 0);
  assert_equals(pixelsNoPreserve[1], 0);
  assert_equals(pixelsNoPreserve[2], 0);
  assert_equals(pixelsNoPreserve[3], 0);
}, "test if OffscreenCanvas.webgl loses context if presereDrawingbuffer is false.");
</script>