first tick should not occur immediatly; no tick should happen after a stop()

+ comments

src/effects.js

@@ -363,17 +363,16 @@ jQuery.fx.prototype = {
 		t.elem = this.elem;
 
 		if ( t() && jQuery.timers.push(t) && !timerId ) {
-			if ( jQuery.support.requestAnimationFrame ) {
-				timerId = true;
-				(function raf() {
+			// Use requestAnimationFrame instead of setInterval if available
+			( timerId = jQuery.support.requestAnimationFrame ) ?
+				window[timerId](function raf() {
+					// timerId will be true as long as the animation hasn't been stopped
 					if (timerId) {
-						window[jQuery.support.requestAnimationFrame](raf);
+						window[timerId](raf);
+						fx.tick();
 					}
-					fx.tick();
-				})();
-			} else {
+				}):
 				timerId = setInterval(fx.tick, fx.interval);
-			}
 		}
 	},