Make the height of the content 95vh and the overflow-y=scroll. Whenever the content on the right is scrolled, the off-canvas menu is unaffected and remains at the top.
CSS:
.mycontent {
height:95vh;
overflow-y:scroll;
/* fix scrolling on webkit touch devices (iPhone etc) */
-webkit-overflow-scrolling: touch;
}
HTML:
<div class="row mycontent" >
<div class="large-12 columns">
<h1>Welcome to Foundation</h1>
</div>
</div>