--- a/dom/canvas/test/captureStream_common.js
+++ b/dom/canvas/test/captureStream_common.js
@@ -37,18 +37,19 @@ CaptureStreamTestHelper.prototype = {
 
   /*
    * Perform the drawing operation on each animation frame until stop is called
    * on the returned object.
    */
   startDrawing: function (f) {
     var stop = false;
     var draw = () => {
+      if (stop) { return; }
       f();
-      if (!stop) { window.requestAnimationFrame(draw); }
+      window.requestAnimationFrame(draw);
     };
     draw();
     return { stop: () => stop = true };
   },
 
   /* Request a frame from the stream played by |video|. */
   requestFrame: function (video) {
     info("Requesting frame from " + video.id);