Gnome 44: GDM background configurable with gsettings

[berglh]

[Description]
The prior implementation of exposing the login screen background setting via `gsettings` is not working as expected in Gnome 44 and Ubuntu 23.04. Specifically, the background image is infact set, however, the default Gnome theme appears as an overlay on top of the com.ubuntu.login-screen schema.

This appears as the default grey background with a remarkably high alpha value of 0.975 from my inspection. If you set a very high contrast background image, you can just make out the outline in beneath the greeter.

My expectation would be that we can set the greeter parameters using the provided gsettings keys, and set the background-color to transparent to only display the configured background-picture-uri key.

gdm@zen:~$ gsettings list-keys com.ubuntu.login-screen
background-color
background-picture-uri
background-repeat
background-size

[Test Case]
1. Boot a desktop session
2. Verify that the default aubergine background is displayed in the greeter.
3. Login as the administrator of the machine
4. Install systemd-container
5. Switch to the GDM user:
  $ sudo machinectl shell gdm@ /bin/bash
6. Apply a different background with the command:
  $ gsettings set com.ubuntu.login-screen background-picture-uri 'file:///usr/share/backgrounds/Lunar-lobster-side_by_Gixo-light.png'
7. Logout
8. Verify that the image 'Lunar-lobster-side_by_Gixo-light.png' is displayed as the background of the greeter.
9. Observe the remaining grey overlay with high alpha value. After login, the background image is briefly displayed without the overlay before initiating the user gnome shell session.

[Further Investigation]
PRATAP-KUMAR has done some excellent work overcoming this with their GitHub project, and it appears that modifying the Gnome theme with this script correctly sets the transparency. However, perhaps there are two layers to the greeter, and even though the background image is set, the additional layer is not exposed for configuration via gsettings.

https://github.com/PRATAP-KUMAR/ubuntu-gdm-set-background/blob/main/ubuntu-gdm-set-background-23.04-transparent#L168-L171

[Launchpad Janitor (janitor)]

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

[Jean-Baptiste Lallement (jibel)]

Setting to high because it is a regression introduced by Gnome 44.

[Sebastien Bacher (seb128)]

Marco, could you investigate when/why that regressed?

[Daniel van Vugt (vanvugt)]

There's also some weird flickering between different grey levels of the background when the login screen starts, and again when it closes to log in. I wonder if that's related.

[Daniel van Vugt (vanvugt)]

This probably went into:

 gnome-shell (45~rc-0ubuntu1) mantic; urgency=medium

but I can't tell because Salsa hasn't been updated recently.