js/theme.js
function getBoundingClientRect(element) {
    var rect = {}; // IE10 10 FIX: Please, don't ask, the element isn't
    // considered in DOM in some circumstances...
    // This isn't reproducible in IE10 compatibility mode of IE11

    try {
      if (isIE(10)) {
        rect = element.getBoundingClientRect();
        var scrollTop = getScroll(element, 'top');
        var scrollLeft = getScroll(element, 'left');
        rect.top += scrollTop;
        rect.left += scrollLeft;
        rect.bottom += scrollTop;
        rect.right += scrollLeft;
      } else {
        rect = element.getBoundingClientRect();
      }
    } catch (e) {}

    var result = {
      left: rect.left,
      top: rect.top,
      width: rect.right - rect.left,
      height: rect.bottom - rect.top
    }; // subtract scrollbar size from sizes

    var sizes = element.nodeName === 'HTML' ? getWindowSizes(element.ownerDocument) : {};
    var width = sizes.width || element.clientWidth || result.right - result.left;
    var height = sizes.height || element.clientHeight || result.bottom - result.top;
    var horizScrollbar = element.offsetWidth - width;
    var vertScrollbar = element.offsetHeight - height; // if an hypothetical scrollbar is detected, we must be sure it's not a `border`
    // we make this check conditional for performance reasons

    if (horizScrollbar || vertScrollbar) {
      var styles = getStyleComputedProperty(element);
      horizScrollbar -= getBordersSize(styles, 'x');
      vertScrollbar -= getBordersSize(styles, 'y');
      result.width -= horizScrollbar;
      result.height -= vertScrollbar;
    }

    return getClientRect(result);
  }
function getBoundingClientRect(element) {
    var rect = {}; // IE10 10 FIX: Please, don't ask, the element isn't
    // considered in DOM in some circumstances...
    // This isn't reproducible in IE10 compatibility mode of IE11

    try {
      if (isIE(10)) {
        rect = element.getBoundingClientRect();
        var scrollTop = getScroll(element, 'top');
        var scrollLeft = getScroll(element, 'left');
        rect.top += scrollTop;
        rect.left += scrollLeft;
        rect.bottom += scrollTop;
        rect.right += scrollLeft;
      } else {
        rect = element.getBoundingClientRect();
      }
    } catch (e) {}

    var result = {
      left: rect.left,
      top: rect.top,
      width: rect.right - rect.left,
      height: rect.bottom - rect.top
    }; // subtract scrollbar size from sizes

    var sizes = element.nodeName === 'HTML' ? getWindowSizes(element.ownerDocument) : {};
    var width = sizes.width || element.clientWidth || result.right - result.left;
    var height = sizes.height || element.clientHeight || result.bottom - result.top;
    var horizScrollbar = element.offsetWidth - width;
    var vertScrollbar = element.offsetHeight - height; // if an hypothetical scrollbar is detected, we must be sure it's not a `border`
    // we make this check conditional for performance reasons

    if (horizScrollbar || vertScrollbar) {
      var styles = getStyleComputedProperty(element);
      horizScrollbar -= getBordersSize(styles, 'x');
      vertScrollbar -= getBordersSize(styles, 'y');
      result.width -= horizScrollbar;
      result.height -= vertScrollbar;
    }

    return getClientRect(result);
  }
Get bounding client rect of given element
@method
@memberof Popper.Utils
@param {HTMLElement} element
@return {Object} client rect