Megamenu expanding on div.menu-sub hover

[ShapesGraphicStudio]

Hello,

The mega menu is expanding when hovering div.menu-sub instead of just opening when hovering parent menu item.

I overrided with "position: fixed" on both #header and #_desktop_top_menu, which seem to cause the issue. If I remove one of the "position: fixed" the issue remains, it's OK when I remove both.

Any idea on what could be wrong here?

Best regards

[ShapesGraphicStudio]

Any idea please?

It can be easily reproduced on the demo with browser devlopper tools by changing l-header to:

.l-header {
    background-color: #fff;
    box-shadow: 0 2px 6px 0 rgba(0,0,0,.2);
    position: fixed;
    z-index: 999;
}