TL;DR: xset s off
“That's easy,” I could hear you think when you read the title. Well, it took me hours to figure it out. It drove me crazy that I'd have to get up and move the mouse every 10 minutes while watching a movie¹, and I couldn't figure out how to disable this behaviour. And when something takes that much effort, I blog about it so that other people might find the solution more easily.
Of course you can easily disable the screen saver through the control panel thing. But I'm not using the Gnome panel, so I never know how to get to that GUI in the first place. Also, you may find, like me, that it just does not work and your screen will still turn black after 10 minutes.
From the console, you can disable the screen saver with:
$ gsettings set org.gnome.desktop.screensaver idle-activation-enabled false
This probably does the same thing as the check box in the GUI. But for me, it wasn't enough: the screen would still blank after 10 minutes!
As it turns out, this is a default built into Xorg itself. Try this:
$ xset q ... Screen Saver: prefer blanking: yes allow exposures: yes timeout: 600 cycle: 600 ...
Yep, that's the problem. Once you've found it, the solution is simple:
$ xset s off $ xset q ... Screen Saver: prefer blanking: yes allow exposures: yes timeout: 0 cycle: 600 ...
This only works until the X server is restarted; put xset s off in your .xsession file to make this change permanent.
¹ At least for some movies. mplayer disables the screensaver automatically, but Flash doesn't.
