ui: Frame skip on JS

internal/ui/ui_js.go

@@ -78,11 +78,21 @@ func shown() bool {
 
 func vsync() {
 	ch := make(chan struct{})
+	// As the performance is generally not good with GopehrJS, consume n 'requestAnimationFrame's
+	// each time.
+	n := 2
+	var l func()
+	l = func() {
+		if 0 < n {
+			n--
 			// TODO: In iOS8, this is called at every 1/30[sec] frame.
 			// Can we use DOMHighResTimeStamp?
-	js.Global.Get("window").Call("requestAnimationFrame", func() {
+			js.Global.Get("window").Call("requestAnimationFrame", l)
+			return
+		}
 		close(ch)
-	})
+	}
+	l()
 	<-ch
 }