Click a main menu item that HAS submenus, the screen shifts upward by about 20 pixels. It’s only on a click… hovering over the same menu item and then moving the mouse down to a submenu and clicking THAT all works fine without any shift.
Clicking a main menu item that does NOT have sub-menus does NOT cause the shift.
this only occurs if you’re slightly scrolled down from the Top of the page, as each of the parent menu URLs are a # which is a jumplink that will return you to the top of the screen.
You can edit each of those Menu Items and delete the # from the URL. So its just an empty field.