I have Blank screen disabled but my PC seems to be ignoring this. This issue also ignores the Caffeine extension. This is pretty recent behavior as it's been working for a long while now.
Disabling idle-dim in dconf seems to have done it.
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This is from another answer I posted recently. You may not need the Login screen section.
To prevent screen from turning off you need two settings, one under battery power the other when plugged into wall outlet (A/C).
gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-battery-timeout <time_in_seconds>
gsettings set org.gnome.settings-daemon.plugins.power sleep-inactive-ac-timeout <time_in_seconds>
So for both Battery and A/C set the time to 0 (never).
gsettings set org.gnome.desktop.screensaver idle-activation-enabled false
In Unix & Linux someone complained when on battery screen dims every 20 seconds and wants to turn that feature off:
gsettings set org.gnome.settings-daemon.plugins.power idle-dim false
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!