gecko/testing/mochitest/tests/SimpleTest/WindowSnapshot.js

netscape.security.PrivilegeManager.enablePrivilege('UniversalXPConnect');

var gWindowUtils;

try {
  gWindowUtils = window.QueryInterface(CI.nsIInterfaceRequestor).getInterface(CI.nsIDOMWindowUtils);
  if (gWindowUtils && !gWindowUtils.compareCanvases)
    gWindowUtils = null;
} catch (e) {
  gWindowUtils = null;
}

function snapshotWindow(win) {
  var el = document.createElementNS("http://www.w3.org/1999/xhtml", "canvas");
  el.width = win.innerWidth;
  el.height = win.innerHeight;

  // drawWindow requires privileges
  netscape.security.PrivilegeManager.enablePrivilege('UniversalXPConnect');

  el.getContext("2d").drawWindow(win, win.scrollX, win.scrollY,
				 win.innerWidth, win.innerHeight,
				 "rgb(255,255,255)");
  return el;
}

// If the two snapshots aren't equal, returns their serializations as data URIs.
function compareSnapshots(s1, s2) {
  netscape.security.PrivilegeManager.enablePrivilege('UniversalXPConnect');

  var s1Str, s2Str;
  var equal = false;
  if (gWindowUtils) {
    equal = (gWindowUtils.compareCanvases(s1, s2, {}) == 0);
  }

  if (!equal) {
    s1Str = s1.toDataURL();
    s2Str = s2.toDataURL();

    if (!gWindowUtils) {
      equal = (s1Str == s2Str);
    }
  }

  return [equal, s1Str, s2Str];
}