Fixed elements (eg. navigation menu, modal dialog) in ios that prevents background scrolling.

iosFixedElement.js

// "fixed-element" is the class of the overlay (fixed element) what has "position: fixed"
// Call disableScroll() and enableScroll() to toggle

var freeze = function(e) {
  if (!document.getElementsByClassName("fixed-element")[0].contains(e.target)) {
    e.preventDefault();
  }
}

var disableScroll = function() {
  document.body.style.overflow = "hidden"; // Or toggle using class: document.body.className += "overflow-hidden-class";

  // Only accept touchmove from fixed-element
  document.addEventListener('touchmove', freeze, false);

  // Prevent background scrolling
  document.getElementsByClassName("fixed-element")[0].addEventListener("touchmove", function(e) {
    var top = this.scrollTop,
      totalScroll = this.scrollHeight,
      currentScroll = top + this.offsetHeight;

    if (top === 0 && currentScroll === totalScroll) {
      e.preventDefault();
    } else if (top === 0) {
      this.scrollTop = 1;
    } else if (currentScroll === totalScroll) {
      this.scrollTop = top - 1;
    }
  });
}

var enableScroll = function() {
  document.removeEventListener("touchmove", freeze);
  document.body.style.overflow = "";
}