sousa-gecko/testing/web-platform/tests/common/reftest-wait.js

/**
 * Remove the `reftest-wait` class on the document element.
 * The reftest runner will wait with taking a screenshot while
 * this class is present.
 *
 * See https://web-platform-tests.org/writing-tests/reftests.html#controlling-when-comparison-occurs
 */
function takeScreenshot() {
    document.documentElement.classList.remove("reftest-wait");
}

/**
 * Call `takeScreenshot()` after a delay of at least |timeout| milliseconds.
 * @param {number} timeout - milliseconds
 */
function takeScreenshotDelayed(timeout) {
    setTimeout(function() {
        takeScreenshot();
    }, timeout);
}

/**
 * Ensure that a precondition is met before waiting for a screenshot.
 * @param {bool} condition - Fail the test if this evaluates to false
 * @param {string} msg - Error message to write to the screenshot
 * @returns {bool} True if the condition passed, false if it failed
 */
function failIfNot(condition, msg) {
  const fail = () => {
    (document.body || document.documentElement).textContent = `Precondition Failed: ${msg}`;
    takeScreenshot();
  };
  if (!condition) {
    if (document.readyState == "interactive") {
      fail();
    } else {
      document.addEventListener("DOMContentLoaded", fail, false);
    }
    return false;
  }
  return true;
}

/**
 * Once a text track cue becomes active, pause the video, wait
 * for layout to update, then call takeScreenshot().
 */
function waitForActiveCueAndTakeScreenshot() {
    var videoElement = document.querySelector("video");
    var trackElement = document.querySelector("track");

    if (!failIfNot(videoElement, "Video element not found"))
        return;

    if (!failIfNot(trackElement, "Track element not found"))
        return;

    var textTrack = trackElement.track;

    function pauseVideoAndTakeScreenshot() {
        if (videoElement.paused)
            requestAnimationFrame(() => takeScreenshot());
        else {
            videoElement.addEventListener("pause", function() {
                requestAnimationFrame(() => takeScreenshot());
            });
            videoElement.pause();
        }
    }

    textTrack.oncuechange = function() {
        if (textTrack.activeCues && textTrack.activeCues.length) {
            textTrack.oncuechange = null;
            pauseVideoAndTakeScreenshot();
        }
    };

    if (textTrack.activeCues && textTrack.activeCues.length)
        pauseVideoAndTakeScreenshot();
}