prevent-scroll-propagation.js

var target = document.querySelector('.modal-content');

// Cross-browser listeners: `mousewheel DOMMouseScroll`
window.addEventListener('mousewheel', function(event) {
  var elHeight = target.clientHeight;
  var scrollTop = target.scrollTop;
  var scrollHeight = target.scrollHeight;

  if ((event.deltaY < 0 && scrollTop == 0 ) ||
    (event.deltaY > 0 && scrollTop + elHeight >= scrollHeight)) {
    event.preventDefault();
    return;
  }
}, false);