Answer by LowFieldTheory for Prevent BODY from scrolling when a modal is opened

Since for me this problem presented mainly on iOS, I provide the code to fix that only on iOS:

  if(!!navigator.platform && /iPad|iPhone|iPod/.test(navigator.platform)) {
    var $modalRep    = $('#modal-id'),
        startScrollY = null, 
        moveDiv;  

    $modalRep.on('touchmove', function(ev) {
      ev.preventDefault();
      moveDiv = startScrollY - ev.touches[0].clientY;
      startScrollY = ev.touches[0].clientY;
      var el = $(ev.target).parents('#div-that-scrolls');
      // #div-that-scrolls is a child of #modal-id
      el.scrollTop(el.scrollTop() + moveDiv);
    });

    $modalRep.on('touchstart', function(ev) {
      startScrollY = ev.touches[0].clientY;
    });
  }