Comment 0 for bug 1686257

Impact
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If a person installs gnome-shell (and not ubuntu-gnome-desktop) on Ubuntu and choose gdm3, they will be unable to log in.

Workaround: install ubuntu-gnome-default-settings

Test Case
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From Ubuntu (Unity), run
sudo apt install gdm3

Choose gdm3 when prompted.
Restart.
You should get a login screen.

Regression Potential
--------------------

Other Info
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ubuntu-settings contains this in /usr/share/glib-2.0/10_ubuntu-settings.gchema.override

[org.gnome.desktop.session]
session-name="ubuntu"

gdm3 fails to start unless session-name="gnome"