At first load, start and end markers are placed all together at the top of viewport. This is a first load screenshot:     So, all animations start at the first load.   Then, when the window is resized, the markers goes to its right place. This is a screenshot after resize window:     When I try to do the same in a isolated codepen (see attached codepen) it works as expected. The wrong behavior just happens in my app. I know that is hard to help w