In GNOME’s power management for screens, there are two different notions:
- blanking involves blanking the screen’s contents and powering down the screen (when possible) — nothing is visible any more;
- dimming involves lowering the screen’s backlight (when possible) — the screen’s contents remain visible.
“Dim screen when inactive” is only supposed to control dimming. It works as follows (see the implementation):
- if the screen saver is active, the screen isn’t dimmed;
- if the system isn’t running on battery, the screen isn’t dimmed;
- if the battery is low, the screen is dimmed after 15 seconds (even if it’s disabled!);
- if dimming is enabled
- if an idle delay is defined, it is adjusted to calculate a dimming delay (4/5ths of the idle delay)
- otherwise, the dimming delay is 60 seconds;
- if the idle delay is less than 10 seconds, it is ignored;
- if the system has been idle long enough, the screen is dimmed.
So you’ll only ever see dimming on systems running on battery. The toggle itself only determines whether the screen is dimmed if the battery isn’t low, and the screen will then only dim for a short while before the screen blanks entirely.
(It is possible to see some variants to the behaviour above in edge cases, because the conditions are evaluated when the timer is set, not when it fires; so you can have a timer set while the system is running on battery, and it will fire and the screen will dim even if the system is connected to an external source in the mean time.)