iOS has default scrolling animation on Safari. When you flick the website up or down, it keeps going for a sec, then slow down & stop. This animation stop work when you use overflow:hidden. But you can add it back. Here is the code
html,body {
width: 100%;
height: 100%;
margin: 0px;
padding: 0px;
overflow-x: hidden;
overflow-y: scroll; /* must be scroll */
-webkit-overflow-scrolling: touch;
}