Source: misc/displace.js

/**
 * @file
 * Manages elements that can offset the size of the viewport.
 *
 * Measures and reports viewport offset dimensions from elements like the
 * toolbar that can potentially displace the positioning of other elements.
 */

/**
 * @typedef {object} Drupal~displaceOffset
 *
 * @prop {number} top
 * @prop {number} left
 * @prop {number} right
 * @prop {number} bottom
 */

/**
 * Triggers when layout of the page changes.
 *
 * This is used to position fixed element on the page during page resize and
 * Toolbar toggling.
 *
 * @event drupalViewportOffsetChange
 */

(function ($, Drupal, debounce) {

  'use strict';

  /**
   * @name Drupal.displace.offsets
   *
   * @type {Drupal~displaceOffset}
   */
  var offsets = {
    top: 0,
    right: 0,
    bottom: 0,
    left: 0
  };

  /**
   * Registers a resize handler on the window.
   *
   * @type {Drupal~behavior}
   */
  Drupal.behaviors.drupalDisplace = {
    attach: function () {
      // Mark this behavior as processed on the first pass.
      if (this.displaceProcessed) {
        return;
      }
      this.displaceProcessed = true;

      $(window).on('resize.drupalDisplace', debounce(displace, 200));
    }
  };

  /**
   * Informs listeners of the current offset dimensions.
   *
   * @function Drupal.displace
   *
   * @prop {Drupal~displaceOffset} offsets
   *
   * @param {bool} [broadcast]
   *   When true or undefined, causes the recalculated offsets values to be
   *   broadcast to listeners.
   *
   * @return {Drupal~displaceOffset}
   *   An object whose keys are the for sides an element -- top, right, bottom
   *   and left. The value of each key is the viewport displacement distance for
   *   that edge.
   *
   * @fires event:drupalViewportOffsetChange
   */
  function displace(broadcast) {
    offsets = Drupal.displace.offsets = calculateOffsets();
    if (typeof broadcast === 'undefined' || broadcast) {
      $(document).trigger('drupalViewportOffsetChange', offsets);
    }
    return offsets;
  }

  /**
   * Determines the viewport offsets.
   *
   * @return {Drupal~displaceOffset}
   *   An object whose keys are the for sides an element -- top, right, bottom
   *   and left. The value of each key is the viewport displacement distance for
   *   that edge.
   */
  function calculateOffsets() {
    return {
      top: calculateOffset('top'),
      right: calculateOffset('right'),
      bottom: calculateOffset('bottom'),
      left: calculateOffset('left')
    };
  }

  /**
   * Gets a specific edge's offset.
   *
   * Any element with the attribute data-offset-{edge} e.g. data-offset-top will
   * be considered in the viewport offset calculations. If the attribute has a
   * numeric value, that value will be used. If no value is provided, one will
   * be calculated using the element's dimensions and placement.
   *
   * @function Drupal.displace.calculateOffset
   *
   * @param {string} edge
   *   The name of the edge to calculate. Can be 'top', 'right',
   *   'bottom' or 'left'.
   *
   * @return {number}
   *   The viewport displacement distance for the requested edge.
   */
  function calculateOffset(edge) {
    var edgeOffset = 0;
    var displacingElements = document.querySelectorAll('[data-offset-' + edge + ']');
    var n = displacingElements.length;
    for (var i = 0; i < n; i++) {
      var el = displacingElements[i];
      // If the element is not visible, do consider its dimensions.
      if (el.style.display === 'none') {
        continue;
      }
      // If the offset data attribute contains a displacing value, use it.
      var displacement = parseInt(el.getAttribute('data-offset-' + edge), 10);
      // If the element's offset data attribute exits
      // but is not a valid number then get the displacement
      // dimensions directly from the element.
      if (isNaN(displacement)) {
        displacement = getRawOffset(el, edge);
      }
      // If the displacement value is larger than the current value for this
      // edge, use the displacement value.
      edgeOffset = Math.max(edgeOffset, displacement);
    }

    return edgeOffset;
  }

  /**
   * Calculates displacement for element based on its dimensions and placement.
   *
   * @param {HTMLElement} el
   *   The jQuery element whose dimensions and placement will be measured.
   *
   * @param {string} edge
   *   The name of the edge of the viewport that the element is associated
   *   with.
   *
   * @return {number}
   *   The viewport displacement distance for the requested edge.
   */
  function getRawOffset(el, edge) {
    var $el = $(el);
    var documentElement = document.documentElement;
    var displacement = 0;
    var horizontal = (edge === 'left' || edge === 'right');
    // Get the offset of the element itself.
    var placement = $el.offset()[horizontal ? 'left' : 'top'];
    // Subtract scroll distance from placement to get the distance
    // to the edge of the viewport.
    placement -= window['scroll' + (horizontal ? 'X' : 'Y')] || document.documentElement['scroll' + (horizontal) ? 'Left' : 'Top'] || 0;
    // Find the displacement value according to the edge.
    switch (edge) {
      // Left and top elements displace as a sum of their own offset value
      // plus their size.
      case 'top':
        // Total displacement is the sum of the elements placement and size.
        displacement = placement + $el.outerHeight();
        break;

      case 'left':
        // Total displacement is the sum of the elements placement and size.
        displacement = placement + $el.outerWidth();
        break;

      // Right and bottom elements displace according to their left and
      // top offset. Their size isn't important.
      case 'bottom':
        displacement = documentElement.clientHeight - placement;
        break;

      case 'right':
        displacement = documentElement.clientWidth - placement;
        break;

      default:
        displacement = 0;
    }
    return displacement;
  }

  /**
   * Assign the displace function to a property of the Drupal global object.
   *
   * @ignore
   */
  Drupal.displace = displace;
  $.extend(Drupal.displace, {

    /**
     * Expose offsets to other scripts to avoid having to recalculate offsets.
     *
     * @ignore
     */
    offsets: offsets,

    /**
     * Expose method to compute a single edge offsets.
     *
     * @ignore
     */
    calculateOffset: calculateOffset
  });

})(jQuery, Drupal, Drupal.debounce);