This works:

mlindholm:

My Stowaway Sierra keyboard arrived today, and in my poking around (and reading other suggestions) I think I found a hack that fixes the “Fn” menu issue.

Edit the file /usr/share/X11/xkb/keycodes/xfree86 , and comment out the line:

<MENU> = 117;
so it reads
//<MENU> = 117;