code: function showBackdrop(scope, element, options) {
        if (options.nestLevel) return angular.noop;
  
        // If we are not within a dialog...
        if (options.disableParentScroll && !$mdUtil.getClosest(options.target, 'MD-DIALOG')) {
          // !! DO this before creating the backdrop; since disableScrollAround()
          //    configures the scroll offset; which is used by mdBackDrop postLink()
          options.restoreScroll = $mdUtil.disableScrollAround(options.element, options.parent);
        } else {
          options.disableParentScroll = false;
        }
  
        if (options.hasBackdrop) {
          options.backdrop = $mdUtil.createBackdrop(scope, "md-menu-backdrop md-click-catcher");
  
          $animate.enter(options.backdrop, $document[0].body);
        }
  
        /**
         * Hide and destroys the backdrop created by showBackdrop()
         */
        return function hideBackdrop() {
          if (options.backdrop) options.backdrop.remove();
          if (options.disableParentScroll) options.restoreScroll();
        };
      }
docs: Show modal backdrop element...
  @returns {function(): void} A function that removes this backdrop