The system installed from Ubuntu-MATE 20190829.1 ISO has GNOME Shell and GDM3 as default

[Norbert]

Steps to reproduce:
1. Download Ubuntu-MATE 20190829.1 daily ISO (http://cdimage.ubuntu.com/ubuntu-mate/daily-live/20190829.1/eoan-desktop-amd64.iso, MD5 199cb3158ddf4b5ad7f4c24f9a3a888d)
2. Boot ISO with default settings
3. Find Install Ubuntu launcher and start it, wait installation to finish

Expected result:
* freshly installed system has lightdm and MATE session

Actual result:
* freshly installed system has GDM3 and GNOME Shell sessions.
* GDM3 is set to default.

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 5.2.0-13.14-generic 5.2.8
Uname: Linux 5.2.0-13-generic x86_64
ApportVersion: 2.20.11-0ubuntu7
Architecture: amd64
CurrentDesktop: MATE
Date: Thu Aug 29 18:29:39 2019
InstallCmdLine: file=/cdrom/preseed/ubuntu-mate.seed initrd=/casper/initrd quiet splash --- maybe-ubiquity
InstallationDate: Installed on 2019-08-29 (0 days ago)
InstallationMedia: Ubuntu-MATE 19.10 "Eoan Ermine" - Alpha amd64 (20190829.1)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Norbert (nrbrtx)]

Some debug info:

$ pstree
systemd─┬─ModemManager───2*[{ModemManager}]
        ├─NetworkManager───2*[{NetworkManager}]
        ├─accounts-daemon───2*[{accounts-daemon}]
        ├─acpid
        ├─anacron
        ├─avahi-daemon───avahi-daemon
        ├─colord───2*[{colord}]
        ├─cron
        ├─cups-browsed───2*[{cups-browsed}]
        ├─cupsd
        ├─dbus-daemon
        ├─gdm3─┬─gdm-session-wor─┬─gdm-x-session─┬─Xorg───{Xorg}
        │ │ │ ├─gnome-session-b───2*[{gnome-+
        │ │ │ └─2*[{gdm-x-session}]
        │ │ └─2*[{gdm-session-wor}]
        │ └─2*[{gdm3}]
        ├─gnome-keyring-d───3*[{gnome-keyring-d}]
        ├─2*[kerneloops]
        ├─networkd-dispat
        ├─polkitd───2*[{polkitd}]
        ├─rsyslogd───3*[{rsyslogd}]
        ├─rtkit-daemon───2*[{rtkit-daemon}]
        ├─snapd───12*[{snapd}]
        ├─systemd─┬─(sd-pam)
        │ ├─at-spi-bus-laun─┬─dbus-daemon
        │ │ └─3*[{at-spi-bus-laun}]
        │ ├─at-spi2-registr───2*[{at-spi2-registr}]
        │ ├─dbus-daemon
        │ ├─dconf-service───2*[{dconf-service}]
        │ ├─evolution-addre───5*[{evolution-addre}]
        │ ├─evolution-calen───8*[{evolution-calen}]
        │ ├─evolution-sourc───3*[{evolution-sourc}]
        │ ├─firefox─┬─3*[Web Content───19*[{Web Content}]]
        │ │ ├─Web Content───21*[{Web Content}]
        │ │ ├─WebExtensions───19*[{WebExtensions}]
        │ │ └─61*[{firefox}]
        │ ├─gnome-session-b─┬─agent───2*[{agent}]
        │ │ ├─applet.py
        │ │ ├─blueman-applet───3*[{blueman-applet}]
        │ │ ├─evolution-alarm───5*[{evolution-alarm}]
        │ │ ├─mate-maximus───3*[{mate-maximus}]
        │ │ ├─mate-power-mana───3*[{mate-power-mana}]
        │ │ ├─mate-screensave───3*[{mate-screensave}]
        │ │ ├─mate-settings-d───4*[{mate-settings-d}]
        │ │ ├─nm-applet───3*[{nm-applet}]
        │ │ ├─polkit-mate-aut───3*[{polkit-mate-aut}]
        │ │ ├─update-notifier───3*[{update-notifier}]
        │ │ └─3*[{gnome-session-b}]
        │ ├─gnome-session-c───{gnome-session-c}
        │ ├─gnome-shell─┬─ibus-daemon─┬─ibus-dconf───3*[{ibus-dconf}]
        │ │ │ ├─ibus-engine-sim───2*[{ibus-engi+
        │ │ │ ├─ibus-extension-───3*[{ibus-exte+
        │ │ │ └─2*[{ibus-daemon}]
        │ │ ├─sh───pxgsettings───3*[{pxgsettings}]
        │ │ └─8*[{gnome-shell}]
        │ ├─gnome-shell-cal───5*[{gnome-shell-cal}]
        │ ├─goa-daemon───3*[{goa-daemon}]
        │ ├─goa-identity-se───2*[{goa-identity-se}]
        │ ├─gsd-a11y-settin───3*[{gsd-a11y-settin}]
        │ ...

[Steve Langasek (vorlon)]

mate-notification-daemon Provides: notification-daemon. This isn't a bug in update-notifier.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Martin Wimpress  (flexiondotorg)]

From the germinate output:

```
* Chose gnome-shell to satisfy libnotify4
```

  * https://people.canonical.com/~ubuntu-archive/germinate-output/ubuntu-mate.eoan/_germinate_output

[Martin Wimpress  (flexiondotorg)]

Fixed via ubuntu-mate-meta 1.249.

xinit was pulling in gnome-terminal, Adding xterm to the Ubuntu MATE seeds and rebuilding ubuntu-mate-meta has resolved the issue.

I don't actually understand why though, because xinit has `Recommends: xserver-xorg | xserver, gnome-terminal | xterm | x-session-manager | x-window-manager | x-terminal-emulator`

  * mate-session-manager provides x-session-manager
  * marco (MATE Window manager) provides x-window-manager
  * mate-terminal provides x-terminal-emulator

So why did seeding xterm fix this?