So comparing this laptop running Ubuntu 26.04 to another running 24.04,
changing the brightness settings at the GDM greeter on both causes that
brightness to apply to the user session that logs in. I suppose it makes
sense that display brightness isn't the same as any other personal
preference, since you can potentially use multiple displays or switch
displays during your session.

So the main difference -- and the source of my annoyance -- is that the
GDM session had the automatic screen brightness enabled in its
configuration.

I didn't see a way to turn the ambient light sensor off in the BIOS. I
saw Daniel's message regarding copying personal dconf settings, but I
wanted to try resolving this problem in a less heavy-handed way.

I created a dconf profile config for GDM:

```
# /etc/dconf/profile/gdm

user-db:user
system-db:gdm
file-db:/usr/share/gdm/greeter-dconf-defaults
```

Then I created system-wide setting turning off autoomatic brightness for
GDM:

```
# /etc/dconf/db/gdm.d/99-brightness

[org/gnome/settings-daemon/plugins/power]
ambient-enabled=false
```

Then I ran `sudo dconf update` and rebooted my machine.

On restart, GDM is running, and it's no longer automatically switching
the display brightness (at least as far as I can tell).

I'm not sure if this is the correct method for doing what I want. It
seems to work, at least.

It's a little irritating that I have to do this via config files and
command line interaction, but it sounds like there's a long-standing
feature request for this with upstream already.

Unless there's some downside with the approach that I took, we can
probably close this out.

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https://bugs.launchpad.net/bugs/2147357

Title:
  Brightness settings from GNOME login manager override GNOME session
  brightness settings

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