--- a/toolkit/components/gfx/SanityTest.js
+++ b/toolkit/components/gfx/SanityTest.js
@@ -41,31 +41,24 @@ function reportResult(val) {
     let histogram = Services.telemetry.getHistogramById("GRAPHICS_SANITY_TEST");
     histogram.add(val);
   } catch (e) {}
 
   Preferences.set(RUNNING_PREF, false);
   Services.prefs.savePrefFile(null);
 }
 
-function takeWindowSnapshot(win) {
+function takeWindowSnapshot(win, ctx) {
   // Take a snapshot of the window contents, and then close the window
-  var canvas = win.createElementNS("http://www.w3.org/1999/xhtml", "canvas");
-  canvas.setAttribute("width", PAGE_WIDTH);
-  canvas.setAttribute("height", PAGE_HEIGHT);
-
+  //
   // TODO: drawWindow reads back from the gpu's backbuffer, which won't catch issues with presenting
   // the front buffer via the window manager. Ideally we'd use an OS level API for reading back
   // from the desktop itself to get a more accurate test.
-  var ctx = canvas.getContext("2d");
   var flags = ctx.DRAWWINDOW_DRAW_CARET | ctx.DRAWWINDOW_DRAW_VIEW | ctx.DRAWWINDOW_USE_WIDGET_LAYERS;
   ctx.drawWindow(win.ownerGlobal, 0, 0, PAGE_WIDTH, PAGE_HEIGHT, "rgb(255,255,255)", flags);
-
-  win.ownerGlobal.close();
-  return ctx;
 }
 
 // Verify that all the 4 coloured squares of the video
 // render as expected (with a tolerance of 64 to allow for
 // yuv->rgb differences between platforms).
 //
 // The video is 64x64, and is split into quadrants of
 // different colours. The top left of the video is 8,72
@@ -77,28 +70,52 @@ function takeWindowSnapshot(win) {
 // we don't want to fail unnecessarily.
 function verifyVideoRendering(ctx) {
   return testPixel(ctx, 18, 82, 255, 255, 255, 255, 64) &&
     testPixel(ctx, 50, 82, 0, 255, 0, 255, 64) &&
     testPixel(ctx, 18, 114, 0, 0, 255, 255, 64) &&
     testPixel(ctx, 50, 114, 255, 0, 0, 255, 64);
 }
 
-function testGfxFeatures(event) {
-  var win = event.target;
-  var canvas = takeWindowSnapshot(win);
+function testCompositor(win, ctx) {
+  takeWindowSnapshot(win, ctx);
 
-  if (!verifyVideoRendering(canvas)) {
+  if (!verifyVideoRendering(ctx)) {
     reportResult(TEST_FAILED_VIDEO);
     Preferences.set(DISABLE_VIDEO_PREF, true);
-  } else {
-    reportResult(TEST_PASSED);
+    return false;
   }
+
+  reportResult(TEST_PASSED);
+  return true;
 }
 
+let listener = {
+  QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver]),
+
+  win: null,
+  canvas: null,
+
+  scheduleTest: function(win) {
+    this.win = win;
+    this.win.onload = this.onWindowLoaded.bind(this);
+  },
+
+  onWindowLoaded: function() {
+    this.canvas = this.win.document.createElementNS("http://www.w3.org/1999/xhtml", "canvas");
+    this.canvas.setAttribute("width", PAGE_WIDTH);
+    this.canvas.setAttribute("height", PAGE_HEIGHT);
+    this.ctx = this.canvas.getContext("2d");
+
+    testCompositor(this.win, this.ctx);
+
+    this.win.ownerGlobal.close();
+  },
+};
+
 function SanityTest() {}
 SanityTest.prototype = {
   classID: Components.ID("{f3a8ca4d-4c83-456b-aee2-6a2cbf11e9bd}"),
   QueryInterface: XPCOMUtils.generateQI([Ci.nsIObserver,
                                          Ci.nsISupportsWeakReference]),
 
   shouldRunTest: function() {
     // Only test gfx features if firefox has updated, or if the user has a new
@@ -138,13 +155,13 @@ SanityTest.prototype = {
     if (!this.shouldRunTest()) return;
 
     // Open a tiny window to render our test page, and notify us when it's loaded
     var sanityTest = Services.ww.openWindow(null,
         "chrome://gfxsanity/content/sanitytest.html",
         "Test Page",
         "width=" + PAGE_WIDTH + ",height=" + PAGE_HEIGHT + ",chrome,titlebar=0,scrollbars=0",
         null);
-    sanityTest.onload = testGfxFeatures;
+    listener.scheduleTest(sanityTest);
   },
 };
 
 this.NSGetFactory = XPCOMUtils.generateNSGetFactory([SanityTest]);