Some Ubuntu MATE users report incompatibilities between Ubuntu MATE 20.10 login screen and other window managers as i3. This question was asked today on AskUbuntu.
Technically it is a bug of arctica-greeter. It ignores selection of i3 and loads MATE instead.
I have reported the bug 1910279 to LaunchPad and upstream.
This bug is not only i3-related, other desktops like Xfce are affected too.
The workaround is to revert our latest change: replace Arctica Greeter back to Slick Greeter.
sudo apt autopurge arctica-greeter
then install Slick Greeter back with
sudo apt-get install slick-greeter
then apply green MATE theme from Ubuntu MATE 20.04 LTS to it by executing long single command
cat <<EOF | sudo tee /usr/share/glib-2.0/schemas/31_ubuntu-mate.gschema.override
[x.dm.slick-greeter]
background='/usr/share/backgrounds/ubuntu-mate-common/Green-Wall-Logo.png'
background-color='#3C3B37'
draw-grid=false
draw-user-backgrounds=false
enable-hidpi='auto'
font-name='Ubuntu 11'
icon-theme-name='Ambiant-MATE'
high-contrast=false
show-hostname=true
#logo=Logo file to use
onscreen-keyboard=false
#other-monitors-logo=Logo file to use for other monitors
# play-ready-sound=A sound file to play when the greeter is ready
screen-reader=false
show-power=true
show-a11y=true
show-keyboard=true
show-clock=true
show-quit=true
theme-name='Ambiant-MATE'
xft-antialias=true
xft-dpi=96
xft-hintstyle='hintslight'
xft-rgba='rgb'
EOF
sudo glib-compile-schemas /usr/share/glib-2.0/schemas
then reboot and login to any session.
To revert the above customization use:
sudo rm /usr/share/glib-2.0/schemas/31_ubuntu-mate.gschema.override
sudo glib-compile-schemas /usr/share/glib-2.0/schemas
sudo apt-get install arctica-greeter