Divi fixed mobile menu anchors issues

Hello,
I have some issues with the Theme Divi.
I'm trying to get the header fixed on mobile with the hamburger icon, so i can access the menu at any moment i need on my One Page.
Page is visible here : http://achard-sa.com/actu/
I'm using this in order to fix the header on the top :
@media only screen and ( max-width: 980px ) {
?#?main?-header .container {
width:100%;
box-sizing:border-box;
padding-right:30px;
padding-left:30px;
position:fixed;
background-color: ?#?fff?;
}
When i click the icon, i can see the menu.
But when i click on a menu item, the menu remains on top and i can not read the content behind.
Any help would be appreciated...
Maybe javascript could help, or only CSS?? I don't find the solution and don't know JS at all.
Thank you in advance
Chris