gecko-dev/layout/reftests/scrolling/scrolling.js

var topElements;
var failed = false;

function doScroll(d)
{
  if (failed)
    return;
  for (var i = 0; i < topElements.length; ++i) {
    var e = topElements[i];
    e.scrollTop = d;
    if (e.scrollTop != d) {
      document.documentElement.textContent =
          "Scrolling failed on " + e.tagName + " element, " +
          "tried to scroll to " + d + ", got " + e.scrollTop +
          " (Random number: " + Math.random() + ")";
      failed = true;
    }
  }
}

// bug 1134459, images are decoded off main thread
// Wait for the load event so we know all images have loaded
document.onload = function() {
  topElements = document.getElementsByClassName("scrollTop");
  if (!topElements.length) {
    topElements = [document.documentElement];
  }

  if (document.location.search == '?ref') {
    doScroll(20);
  } else if (document.location.search == '?up') {
    doScroll(40);
    document.documentElement.setAttribute("class", "reftest-wait");
    window.addEventListener("MozReftestInvalidate", function() {
        document.documentElement.removeAttribute("class");
        doScroll(20);
        }, false);
  } else {
    doScroll(1);
    document.documentElement.setAttribute("class", "reftest-wait");
    window.addEventListener("MozReftestInvalidate", function() {
        document.documentElement.removeAttribute("class");
        doScroll(20);
        }, false);
  }
}