Question: I have Blank screen disabled but my PC seems to be ignoring this. This issue also ignores the [Caffeine](https://extensions.gnome.org/extension/517/caffeine/) extension. This is pretty recent behavior as it's been working for a long while now.

[![Power settings](https://i.stack.imgur.com/6nLZm.png)](https://i.stack.imgur.com/6nLZm.png)

Disabling idle-dim in dconf seems to have done it.

Answer: 
Nothing in the accepted answer has worked for me in 18.04. Instead I had to do the following:

```
gsettings set org.gnome.settings-daemon.plugins.power idle-brightness 100

```

Note that this doesn't set the idle to 100% of the max but 100% of the current setting (at least I think that's what it's doing). If you have your screen dimmed already it will stay dimmed.
I just want to add, that as a troubleshooting step w.r.t. my ...

Conspiratorially derived answer, that Ubuntu "Did Not Want" me logged into "Gnome on Xorg" ... as it sure seemed.

After a restart and the issue gone ... again, I specifically logged out of "Ubuntu" and logged back in via "Gnome on Xorg",

and No Problems! Issue APPEARS gone still?? (all other things being equal)

... So what has changed since this morning?

You tell me!