title: how to disable scrolling on background divs in mobile web application
body: <p>in mobile web application I'd often use modals.</p>
  
  <p>Some modals can expand sizes and thus were made scrollable.</p>
  
  <p>How can I enable higher z-indexed modal to be scrollable and set background div not to scroll?</p>
  
  <p>b/c whenever I scroll with two fingers on the current modal, sometimes background scroll, sometimes foreground scrolls.</p>