Dropdown menu does not work smoothly with IE 10. If you go ACROSS the menu slowly, the sub-menu does not drop down.

This is common IE10 bug and it will be in OpenCart default theme too.

This is solution how to solve this issue in OpenCart engine:

Open catalog/view/javascript/common.js:

Replace:

// IE6 & IE7 Fixes
if ($.browser.msie) {
if ($.browser.version <= 6) {
$('#column-left + #column-right + #content, #column-left + #content').css('margin-left', '195px');

$('#column-right + #content').css('margin-right', '195px');

$('.box-category ul li a.active + ul').css('display', 'block');
}

if ($.browser.version <= 7) {
$('#menu > ul > li').bind('mouseover', function() {

With:

// IE6, IE7 & IE10 Fixes
if ($.browser.msie) {
if ($.browser.version <= 6) {
$('#column-left + #column-right + #content, #column-left + #content').css('margin-left', '195px');

$('#column-right + #content').css('margin-right', '195px');

$('.box-category ul li a.active + ul').css('display', 'block');
}

if ($.browser.version <= 7) {
$('#menu > ul > li').bind('mouseover', function() {
$(this).addClass('active');