What if it sends an AJAX request? You can't exactly undo them. The browser JS environment is sufficiently complex that "just undo"ing something seems like it would be... complex.
At this point it's a good idea to keep in mind the effort/gains ratio. You're proposing massive changes to JavaScript engines in order to have double tapping work a certain way. Changing the gesture or tweaking the delays might yield a less optimal solution, but the effort is so much closer to 0. If you were going to invest the effort you're proposing into something, would improving this delay be the best knowable use of that effort?
