To put it short, the mechanics of a horizontal menu and a vertical menu are very different. Making the main menu work in both ways has required some restrictions, especially taking into the consideration all the settings for the main menu.
How to set it?
To set your menu horizontally, simply go to your theme config -> menu.mainmenu.submenu_style, and set it to "horisontal".
When using the horizontal menu, The following settings are set automatically and variations will be ignored from the theme config:
Code
mainmenu.menuback.opacity._value = 0;
mainmenu.menuback.opacity_rollover._value = 0;
mainmenu.horizontal_margin._value = 1;
mainmenu.vertical_margin._value = 1;
mainmenu.autohide._value = false;
mainmenu.close_same_level._value = true;
mainmenu.indent._value = 0;
# Your are yourself responsible for keeping your menu width within boundaries of the screen width.
# In your theme config try setting the following value to get automatic size(width) of the menu item element:
mainmenu.menuitem.minimum_width._value = 0;
# The horizontal menu currently only supports 2 tiers/levels of subfolders. Any deeper levels will not display correctly.