ben Start / stop works fine, but you can not jump to an other position in the timeline.
I have checked myself and can confirm this issue in Safari browser. The problem is related to the popup interface, which uses swipe/drag gestures for navigation. I will need to look into it, why it only affects Safari, and if it can be resolved.
For now, the only workaround would be to disable popup_video option, so that videos open in modal instead of the popup.