Overflow not working on iPhone and iPad

Question

I'm using this javascript part to hide or no scrollbar when my modal is open or no, but iPhone and iPad don't hide my scroll anyway ? The problem comes from position relative ?

here is my javascript code

function openNav()  {
document.getElementById("myNav").style.cssText = "height:100%";
document.body.style.overflow = "hidden";
}
function closeNav() {
document.getElementById("myNav").style.cssText = "height:0%";
document.body.style.overflow = "scroll";
}

Answer 1

to resolve my problem for the modal I need to add

-webkit-overflow-scrolling:touch

in my CSS file here

*, *:after, *:before {

But the problem still exist when i try to open my fullscreen menu !!