tl;dr: display-settings have no memory that a display should stay on whenever it is connected and are now off by default when they are discovered. After the latest upgrade, it seems the display manager settings have got too clever for me. I have a screen connected to both the HDMI and VDI outputs (i have my reasons, but I'd usually keep the VDI off while using xfce) Before, when I unplugged the HDMI port from the screen and plug it to a different device (a projector), the display was turned-on on replug, and it was easily picked by whatever I connected it too. Now, unplugging an output means it automatically gets turned off. Replugging it requires reactivating the output and mirroring mode in the display-settings (or in the dialog that comes up), which means I need to make sure the VDI output is active, so this stuff can be displayed somewhere. Before I just freely connected/disconnected the HDMI cable and things worked because it remembered that HDMI output was supposed to be active whenever it was plugged. Now it falls into the assumption that new displays are off by default and has no memory of what the settings were for the previous, so the user must decide on every occasion. Also, if only one display is active, and the cable from it is disconnected, bad luck, no displays active anymore, regardless of whether the cable is reconnected. This makes sense for laptops where there is always a display that cannot be unplugged, but for every other setup is painful. Any ways to get back to the original behaviours, or workaround this?
When I unplug HDMI/VGA port on my laptop the XFCE display manger changes the Samsung 20" display settings to black and white. When I toggle the display-manager between mirror and laptop I can see the laptop mode is still in color. I have installed kdm3 & kdm4 on this PC. I'm not sure why they are both showing up in the apt list. I'm trying to reinstall the display manager now and will let you know.
I have an ASUS MG279Q display connected via displayport to my laptop with Intel GPU (but got the same behaviour with my GTX670 desktop). I work in a dual monitor environment. When this screen turns off (e.g. when the power manager turns it off after 10 minutes) it seems to briefly disconnect. When I move the mouse with the display settings open on the other screen, I see the ASUS screen briefly disappear and appear again in the display settings window. Xfce *needs* to remember my screen configuration. To enable it again I'd have to go through: 1. Mark the Use this display checkbox 2. Screen makes 'pop' sound (probably speaker amplifier turning on) 3. Screen disappears in the display manager overview 4. Screen appears again (but the state is disabled again). Additionally, the displays screen pops up with the choice of single display, mirror, extend to right etc.) 5. When I am quick enough, I can mark the use this display checkbox again. When I wait too long the screen is back in standby mode and I have to try again. Tested Xubuntu 15.04 (laptop), 15.10(desktop) and Xubuntu daily-live 2015-09-11
I turn off both monitors for lunch break. Since 4.12 they stay black when I turn them on again. Desktop computer. I don't unplug, just turn the monitors off with their on-off button. If I forget to use my workaround I can't go back into my X session.
This appears to be the root cause of a fairly large number of reported "bugs" elsewhere, most of which people work around with elaborate xrandr scripts. I'm running into this on a docked laptop. The observed behavior is that if the monitor is powered off for any reason, it becomes deconfigured; it won't be reactivated until such time as I rerun xrandr to enable it. Now, that might be survivable, albeit annoying... Except that the monitor powers off when screenblanked. So if I'm AFK for an hour or two, the monitor shuts itself off. No problem, it'll come back on when requested... Except that, because XFCE has disabled it, it never gets told to come back on. The expected behavior would be that, on key or mouse input, XFCE would send wakeup signals to any monitors it had attached recently, causing power-saving monitors to wake up again. Or, at the very least, it would remember how they'd been configured, so if I manually turn the display back on, XFCE can restore the display. Instead, XFCE tells the display to power off, then having seen that the display has powered off, it disables the display. It then has no way to ever restore functionality to that display except by having me manually run a script, or cause a script to be run. But that's silly. It's not anyone *else's* job to turn the monitor back on; it's XFCE's, because XFCE is what turned it off. This appears to have survived well over a year and a half. I'm gonna try with a different window manager, because I somehow expect that'll magically make things better.
Seems a duplicate of bug 10459
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