At my display manager (Slim), all my screens are configured exactly as I want them: HDMI-0 on the left and VGA-0 on the right if they’re connected, or LVDS-0 alone if they aren’t.

In previous versions of XFCE (4.10 and below), everything was perfect because that configuration would stay untouched after logging in.

As of 4.12, XFCE decides to turn off VGA-0 every time I log in. No matter what I do in display settings, I can’t convince it to stop doing this, and the control panel also refuses to ever turn VGA-0 back on. I have to use “xrandr --output VGA-0 --auto --right-of HDMI-0” to get it back.

Note that this is NOT bug #8529. That bug claims that you have to change a display setting of some sort before the display activates. In my case, the display never activates even if I do try changing a display setting (e.g. I change the resolution, and while Use This Display remains ticked, the monitor stays off and its icon in the display settings control panel still reads Disabled.

I am using nvidia-drivers, and command-line xrandr has no trouble configuring screens any way I ask it to (within the hardware capabilities, obviously—I can’t enable three screens simultaneously).