When I am using 'night light' functionality under GNOME, and xscreensaver starts, you will see the night light turn off suddenly (in other words, it will go to 'full bright') initially as the screen starts to fade. During the display of the screensaver animation, and after unlocking the screen, night light is now turned off completely. I then have to manually turn night light off then on to get the screen dimmed again. Obviously, this is a bit annoying.

Through a bit of research, I found the cause of this issue – xscreensaver's 'fade' functionality uses the same method as GNOME night light.

It seems like xscreensaver will 'fade' from "full bright" and then restore the screen to "full bright" – it does not seem to back up the original brightness/gamma settings and restore them.

The workaround of disabling 'fade' via .xscreensaver does work. A better fix might be to patch xscreensaver to back up and restore whatever display setting it uses for fading – and also start from the current brightness rather than temporarily bump the brightness to 'full bright' as it starts the fade which looks weird.

It might be fun to try to improve xscreensaver to do this better.