"The main menu in the header can use a down faced arrow, no problem. However, inside the menu, a right arrow is more descriptive of the action because the fly-out opens in that direction. Other software and OSes also follow the right arrow convention inside contextual menus."

See https://forum.xwiki.org/t/menu-macro-changed-lts-14-10-20-to-15-10-5/14072/15