mjhjmtu

It isn't actually locking or going to sleep. After a few seconds, the screen just clicks to a slightly darker setting. I have looked through dconf editor and the settings menu, and can't find any. Someone on here asked a similar question about Fedora, but it was a system update that fixed it. I'm running Debian 9, so that won't work for me. Below I will include a screenshot of my dconf power menu:

To be clear, this issue doesn't plague me when the laptop is plugged in. But when it's unplugged, the computer gets noticeably darker after just a few seconds of idling.

Also, the screen brightens back up as soon as I touch the mouse or the keyboard.

Solved, kind of: While dealing with some other issues, I rebooted my computer a few times (something I hadn't done since messing with dconf) and the issues seems to have gone away. So perhaps it was just necessary to reboot after editing?

Especially on Laptops, the embedded controller (EC) does a lot of things, including power management and controlling of the backlight.

Unfortunately, every vendor does this differently, and none of this is documented somewhere. For Windows, the pre-installed drivers reasonably support it; for Linux, the situation is difficult because of the diversity and lack of documentation.

So I very much suspect it is the EC that turns down your backlight after a few seconds of confirming that there's no power from the power supply. (My laptop does this instantly). If that is the case, looking at Gnome settings or filing a bug is not going to help.

On my laptop, I can turn the brightness up again manually. See if this works on your laptop. If you are lucky, the EC produces an ACPI event (or some sort of event) you can catch with a custom script, and then turn the backlight back up automatically, but how this works (if at all) will depend entirely on your laptop, so you need to do a bit of reading and sleuthing. E.g. try acpi_listen.