Lock screen wallpaper shows desktop wallpaper on Ubuntu 18.04 with Gnome Flashback

Question

I installed the GNOME Flashback desktop environment on Ubuntu 18.04. Changing the desktop background works, but changing the lock screen wallpaper does not work. Instead, the GNOME Flashback lock screen uses the desktop background, although it should be using the user-specified lock screen wallpaper. Below are some images to explain this.

Here, you can see the system settings showing the user-chosen wallpapers for the desktop background and the lock screen:

And here, you can see that the GNOME Flashback lock screen uses the desktop wallpaper instead of the lock screen wallpaper:

I can confirm that this behaviour is on Ubuntu 18.04, because I have seen this happening on other Ubuntu 18.04 computers running GNOME Flashback. This problem does not exist in Ubuntu 20.04.

My question is: is it possible to change this behaviour so that GNOME Flashback uses the lock screen wallpaper instead of the desktop wallpaper?

Answer 1

No. You would need to write patch for gnome-screensaver to use correct wallpaper.

EDIT:
Actually gnome-screensaver already has 33_budgie_support.patch that gives a hackish way to use screensaver wallpaper.

You could try to edit /usr/share/dbus-1/services/org.gnome.ScreenSaver.service file. Open file for editing and change Exec=/usr/bin/gnome-screensaver --no-daemon line to Exec=/bin/sh -c "set env XDG_CURRENT_DESKTOP=Budgie /usr/bin/gnome-screensaver --no-daemon".

Please note that it will affect other things in screensaver, for example, there won't be top bar with clock in lock screen.

This might not always work, there are also two screensaver autostart files in /etc/xdg/autostart that might need similar change. Autostart files can be copied from /etc/xdg/autostart to ~/.config/autostart so you don't need to change system files.