Align level 3 submenu in mega menu?

Some mega menus can be very tall. In that case, expanding a level 3 menu may not leave enough space. When this happens, you should pin the level 3 menu to the top instead.

Paste it into the Internal Stylesheet / CSS box:

ul.children[menulevel="2"] li.child.is-parent.menu-expand {
  position: initial;
}

ul.children[menulevel="2"] li.child.is-parent.menu-expand ul.children[menulevel="3"]{
  top: 0px;
}