I have no idea if this is by design or what, but columns seem to hide overflow. Like, if you had an absolutely positioned item within a box in a column that was wider than the column.
https://codepen.io/chriscoyier/pen/boxyMJ

You can't overflow or z-index or whatever your way out of this.
Real world example:

Forced to make sure columns are wide enough so that submenu doesn't get overlapped by another column.