One-up Bootstrap 3.0.0–3.0.3’s method of accounting for scrollbar width when modal is open.

fixBootstrapModalScrollbar.js

// This function courtesy of lostsource: http://stackoverflow.com/questions/13382516
function getScrollbarWidth() {
    var outer = document.createElement("div");
    outer.style.visibility = "hidden";
    outer.style.width = "100px";
    document.body.appendChild(outer);
    var widthNoScroll = outer.offsetWidth;
    outer.style.overflow = "scroll";
    var inner = document.createElement("div");
    inner.style.width = "100%";
    outer.appendChild(inner);
    var widthWithScroll = inner.offsetWidth;
    outer.parentNode.removeChild(outer);
    return widthNoScroll - widthWithScroll;
}

// Event names are for Bootstrap 3. Adjust for Bootstrap 2.
$(document).ready(function() {
  window.scrollbarWidth = getScrollbarWidth();
  var bodyMarginRight = $("body").css("marginRight");
  $(document).on("show.bs.modal", ".modal", function() {
    if (document.body.scrollHeight > $(window).height()) {
      document.body.style.marginRight = scrollbarWidth + "px";
    }
  }).on("hidden.bs.modal", ".modal", function() {
    document.body.style.marginRight = bodyMarginRight;
  });
});