Comment 2 for bug 911813

We currently check this:

if [ -z "$2" -a -n "$RELEASE_UPGRADE_IN_PROGRESS" ]; then
    db_set shared/default-x-display-manager lightdm
    db_fset shared/default-x-display-manager seen true
else
    db_input high shared/default-x-display-manager || :
    db_go || :
fi

update-manager sets $RELEASE_UPGRADE_IN_PROGRESS, so it seems in your upgrade method this wasn't set. Was your upgrade done using apt-get dist-upgrade? In that case we actually expect the prompt.

If it was from do-release-upgrade, it's meant to not show. We shouldn't have a previously installed lightdm in that case, so the -z "$2" part ought to be okay. Do you happen to have full logs from that upgrade still?

Thanks!