Comment 24 for bug 1576500

I was able to fix the issue between Snappy and XDG_DATA_DIRS by modifying /etc/X11/Xsession.d/60x11-common_xdg_path to read as follows:

# This file is sourced by Xsession(5), not executed.
# Add additionnal xdg paths depending on selected desktop session

DEFAULT_XDG_CONFIG_DIRS='/etc/xdg'
DEFAULT_XDG_DATA_DIRS='/usr/local/share/:/usr/share/'
if [ -n "$DESKTOP_SESSION" ]; then
  # readd default if was empty
  if [ -z "$XDG_CONFIG_DIRS" ]; then
    XDG_CONFIG_DIRS="$DEFAULT_XDG_CONFIG_DIRS"
  fi
  XDG_CONFIG_DIRS="$DEFAULT_XDG_CONFIG_DIRS"/xdg-"$DESKTOP_SESSION":"$XDG_CONFIG_DIRS"
  export XDG_CONFIG_DIRS
  # gnome is already added if gnome-session installed
  if [ "$DESKTOP_SESSION" != "gnome" ]; then
     if [ -z "$XDG_DATA_DIRS" ]; then
       XDG_DATA_DIRS="$DEFAULT_XDG_DATA_DIRS"
     fi
     XDG_DATA_DIRS=/usr/share/"$DESKTOP_SESSION":"$XDG_DATA_DIRS"
     export XDG_DATA_DIRS
  fi
else
  if [ -z "$XDG_CONFIG_DIRS" ]; then
    XDG_CONFIG_DIRS='/etc/xdg'
  fi
  if [ -z "$XDG_DATA_DIRS" ]; then
    XDG_DATA_DIRS='/usr/share:/usr/local/share'
  fi
fi

Notice the "else" section at the end. There is no guarantee that $DESKTOP_SESSION is set, but XDG_DATA_DIRS is still a needed variable.