Ubuntu Desktop will be switching from LightDM to GDM for 17.10. I'm working on handling that transition and trying to sort out the best solution for upgraders that won't negatively impact other flavors.
The initial plan was to drop the current prompt when configuring the package to select which DM to use by default and just switch current lightdm users to gdm3. This would only affect users that have both lightdm and gdm3 installed, but I can see cases where users may have that today. For example an Ubuntu MATE user that has also installed gnome-shell, would have both lightdm and gdm3 installed. Most of the flavors are using lightdm and as far as I know aren't planning to switch to GDM at this time.
We could use a maintainer script to switch the default to gdm3 on upgrade only if ubuntu-session is installed, this would further reduce the impact.
Thoughts, concerns or suggestions all appreciated.
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