Source: misc/dialog/dialog.position.es6.js

/**
 * @file
 * Positioning extensions for dialogs.
 */

/**
 * Triggers when content inside a dialog changes.
 *
 * @event dialogContentResize
 */

(function ($, Drupal, drupalSettings, debounce, displace) {
  // autoResize option will turn off resizable and draggable.
  drupalSettings.dialog = $.extend(
    { autoResize: true, maxHeight: '95%' },
    drupalSettings.dialog,
  );

  /**
   * Position the dialog's center at the center of displace.offsets boundaries.
   *
   * @function Drupal.dialog~resetPosition
   *
   * @param {object} options
   *   Options object.
   *
   * @return {object}
   *   Altered options object.
   */
  function resetPosition(options) {
    const offsets = displace.offsets;
    const left = offsets.left - offsets.right;
    const top = offsets.top - offsets.bottom;

    const leftString = `${
      (left > 0 ? '+' : '-') + Math.abs(Math.round(left / 2))
    }px`;
    const topString = `${
      (top > 0 ? '+' : '-') + Math.abs(Math.round(top / 2))
    }px`;
    options.position = {
      my: `center${left !== 0 ? leftString : ''} center${
        top !== 0 ? topString : ''
      }`,
      of: window,
    };
    return options;
  }

  /**
   * Resets the current options for positioning.
   *
   * This is used as a window resize and scroll callback to reposition the
   * jQuery UI dialog. Although not a built-in jQuery UI option, this can
   * be disabled by setting autoResize: false in the options array when creating
   * a new {@link Drupal.dialog}.
   *
   * @function Drupal.dialog~resetSize
   *
   * @param {jQuery.Event} event
   *   The event triggered.
   *
   * @fires event:dialogContentResize
   */
  function resetSize(event) {
    const positionOptions = [
      'width',
      'height',
      'minWidth',
      'minHeight',
      'maxHeight',
      'maxWidth',
      'position',
    ];
    let adjustedOptions = {};
    let windowHeight = $(window).height();
    let option;
    let optionValue;
    let adjustedValue;
    for (let n = 0; n < positionOptions.length; n++) {
      option = positionOptions[n];
      optionValue = event.data.settings[option];
      if (optionValue) {
        // jQuery UI does not support percentages on heights, convert to pixels.
        if (
          typeof optionValue === 'string' &&
          /%$/.test(optionValue) &&
          /height/i.test(option)
        ) {
          // Take offsets in account.
          windowHeight -= displace.offsets.top + displace.offsets.bottom;
          adjustedValue = parseInt(
            0.01 * parseInt(optionValue, 10) * windowHeight,
            10,
          );
          // Don't force the dialog to be bigger vertically than needed.
          if (
            option === 'height' &&
            event.data.$element.parent().outerHeight() < adjustedValue
          ) {
            adjustedValue = 'auto';
          }
          adjustedOptions[option] = adjustedValue;
        }
      }
    }
    // Offset the dialog center to be at the center of Drupal.displace.offsets.
    if (!event.data.settings.modal) {
      adjustedOptions = resetPosition(adjustedOptions);
    }
    event.data.$element
      .dialog('option', adjustedOptions)
      .trigger('dialogContentResize');
  }

  $(window).on({
    'dialog:aftercreate': function (event, dialog, $element, settings) {
      const autoResize = debounce(resetSize, 20);
      const eventData = { settings, $element };
      if (settings.autoResize === true || settings.autoResize === 'true') {
        $element
          .dialog('option', { resizable: false, draggable: false })
          .dialog('widget')
          .css('position', 'fixed');
        $(window)
          .on('resize.dialogResize scroll.dialogResize', eventData, autoResize)
          .trigger('resize.dialogResize');
        $(document).on(
          'drupalViewportOffsetChange.dialogResize',
          eventData,
          autoResize,
        );
      }
    },
    'dialog:beforeclose': function (event, dialog, $element) {
      $(window).off('.dialogResize');
      $(document).off('.dialogResize');
    },
  });
})(jQuery, Drupal, drupalSettings, Drupal.debounce, Drupal.displace);