Updated version of Supposition. This version is compatible with Superfish 1.6+. See this ancient page for description, example and caveats: http://users.tpg.com.au/j_birch/plugins/superfish/supposition-test/

supposition.js

/*
 * Supposition v0.3a - an optional enhancer for Superfish jQuery menu widget
 *
 * Copyright (c) 2013 Joel Birch - based on work by Jesse Klaasse - credit goes largely to him.
 * Special thanks to Karl Swedberg for valuable input.
 *
 * Dual licensed under the MIT and GPL licenses:
 *   http://www.opensource.org/licenses/mit-license.php
 * 	http://www.gnu.org/licenses/gpl.html
 */

;(function($){

	$.fn.supposition = function(){
		var $w = $(window), /*do this once instead of every onBeforeShow call*/
			_offset = function(dir) {
				return window[dir == 'y' ? 'pageYOffset' : 'pageXOffset']
				|| document.documentElement && document.documentElement[dir=='y' ? 'scrollTop' : 'scrollLeft']
			    || document.body[dir=='y' ? 'scrollTop' : 'scrollLeft'];
			},
			onInit = function(){
				/* I haven't touched this bit - needs work as there are still z-index issues */
				$topNav = $('li',this);
				var cZ=parseInt($topNav.css('z-index')) + $topNav.length;
				$topNav.each(function() {
					$(this).css({zIndex:--cZ});
				});
			},
			onHide = function(){
				this.css({marginTop:'',marginLeft:''});
			},
			onBeforeShow = function(){
				this.each(function(){
					var $u = $(this);
					$u.css('display','block');
					var menuWidth = $u.width(),
						parentWidth = $u.parents('ul').width(),
						totalRight = $w.width() + _offset('x'),
						menuRight = $u.offset().left + menuWidth;
					if (menuRight > totalRight) {
						$u.css('margin-left', ($u.parents('ul').length == 1 ? totalRight - menuRight : -(menuWidth + parentWidth)) + 'px');
					}

					var windowHeight = $w.height(),
						offsetTop = $u.offset().top,
						menuHeight = $u.height(),
						baseline = windowHeight + _offset('y');
					var expandUp = (offsetTop + menuHeight > baseline);
					if (expandUp) {
						$u.css('margin-top',baseline - (menuHeight + offsetTop));
					}
					$u.css('display','none');
				});
			};
		
		return this.each(function() {
			var $this = $(this),
				o = $this.data('sf-options'); /* get this menu's options */
			
			/* if callbacks already set, store them */
			var _onInit = o.onInit,
				_onBeforeShow = o.onBeforeShow,
				_onHide = o.onHide;
				
			$.extend($this.data('sf-options'),{
				onInit: function() {
					onInit.call(this); /* fire our Supposition callback */
					_onInit.call(this); /* fire stored callbacks */
				},
				onBeforeShow: function() {
					onBeforeShow.call(this); /* fire our Supposition callback */
					_onBeforeShow.call(this); /* fire stored callbacks */
				},
				onHide: function() {
					onHide.call(this); /* fire our Supposition callback */
					_onHide.call(this); /* fire stored callbacks */
				}
			});
		});
	};

})(jQuery);

Never mind. It was a wordpress issue. Works great!

@cemelmaci Commenting out this line $u.css('display','none'); solves the flickering issue, but it also disables any animaion on the menu.

this plugin helped me to display sub menu items in the left side, if there is not enough screen space on the right
check out this post https://stackoverflow.com/a/47286812/202503

for anyone that have issue with flashing and animation, replace onBeforeShow this only check for sub-menu and no need to set display to block to check.

onBeforeShow = function () {
                this.each(function () {
                    var $u = $(this);
                    var menuWidth = $u.width(),
                        parentWidth = $u.parents('ul').width(),
                        totalRight = $w.width() + _offset('x'),
                        menuRight = $u.offset().left + menuWidth;
                    if ($u.parents('li').hasClass('menu-has-children')) {
                        menuRight = $u.parents('ul').offset().left + menuWidth + $u.parents('ul').width()
                    }
                    if (menuRight > totalRight) {
                        $u.css('margin-left', ($u.parents('ul').length == 1 ? totalRight - menuRight : -(menuWidth + parentWidth)) + 'px');
                    }

                    var windowHeight = $w.height(),
                        offsetTop = $u.offset().top,
                        menuHeight = $u.height(),
                        baseline = windowHeight + _offset('y');
                    var expandUp = (offsetTop + menuHeight > baseline);
                    if (expandUp) {
                        $u.css('margin-top', baseline - (menuHeight + offsetTop));
                    }
                });
            };