Public bug reported:
### Bug description
I want to turn off the LCD screen and its LED backlight to prevent it from
wearing out too quickly and to save electricity. But I see that the screen
turns back on, a few seconds later. It's subtle, because the screen displays a
100% black image, but I see that the LED backlight is still on, and therefore
the LCD too, which displays black. I can see it when I turn off the room
lighting around me and then look at the screen carefully... It's not a real
total black, just a kind of very dark gray...
This happens in several situations. Either with the screen lock keyboard
shortcut (Ctrl+L), or by clicking on the lock icon in the quick
settings, or with the Gnome GUI Settings > Power > Power Saving > Screen
Blank: xx minutes.
I guess there is a problem with the Display Power Management Signaling
(DPMS) at the root of all this.!?
### To Reproduce
1. Lock your screen: press Super+L, or click on the screen lock icon,
or `dbus-send --type=method_call --dest=org.gnome.ScreenSaver
/org/gnome/ScreenSaver org.gnome.ScreenSaver.Lock`
2. See your desktop background picture blurred with the time displayed
in large letters on top, then turning black about a second later.
3. Turn off the lights around you and look at your screen carefully: **it
displays black but is not really OFF.**
### Expected behavior
I would expect the LED backlight and the LCD display to be off when the
screen blanks or locks. Because I don't want my LCD-LED screen to wear
out too quickly. These screens tend to deteriorate over time. Turning
them off helps preserve colour accuracy, and also saves electricity.
Check my screenshot: "Turn the screen OFF after a period of inactivity".
That doesn't work either.
### Desktop (please complete the following information):
```
System:
Host: HP-EliteDesk Kernel: 6.8.0-45-generic arch: x86_64 bits: 64
Desktop: GNOME v: 46.0 Distro: Ubuntu 24.04.1 LTS (Noble Numbat)
Graphics:
Device-1: Intel HD Graphics 530 driver: i915 v: kernel
Display: wayland server: X.Org v: 23.2.6 with: Xwayland v: 23.2.6
compositor: gnome-shell driver: X: loaded: modesetting unloaded: fbdev,vesa
dri: iris gpu: i915 resolution: 1920x1080~100Hz
API: EGL v: 1.5 drivers: iris,swrast
platforms: wayland,x11,surfaceless,device
API: OpenGL v: 4.6 compat-v: 4.5 vendor: intel mesa v: 24.0.9-0ubuntu0.1
renderer: Mesa Intel HD Graphics 530 (SKL GT2)
```
Screen display connection: DisplayPort (DP) to HDMI
Screen display brand/model: LG 24" FHD 100Hz 5ms IPS Monitor (24MR400)
### Additional context
In addition to my regular Wayland sessions, I also did a quick test on a
"classic" Xorg session. From there, I could do the classic:
```
sleep 1; xset dpms force off
```
Well, it bounces back all lit up a few seconds later. And now the screen isn't
black, I can see my desktop and windows.
I found a workaround. In my screen settings, I set: Input > Auto Input
Switch: off. This means the screen won't try to switch from HDMI to
D-SUB. When set that way, the screen stays off, as long as I don't touch
the mouse or keyboard.
I found that DPMS will sometimes work well, and sometimes not. And it
depends on the monitor settings. It's reproducible, constant, on my
monitor.
Anyway, the problem seems to come from an exchange of signals between
the monitor and the computer. I guess it has to do with the DDC?
In any case, it's neither the computer nor the screen that is defective.
On Windows 10, the same set works as it should. And then on Ubuntu, at
least, when I put the computer to sleep, the screen turns off
automatically, without fail, every time.
### One more thing
I reset my screen to factory settings, i.e. auto-switching between HDMI and
D-SUB. Then I log into a "classic" Xorg session and do this:
```
sleep 1; xset dpms force off; sleep 10; xset q | grep "Monitor is"
Monitor is Off
```
Well, that's not true! Monitor is back on, obviously. And then I just touch the
keyboard to type this:
```
xset q | grep "Monitor is"
Monitor is On
```
Yes, "Monitor is On"...
P.S.: I think many people are affected. I saw it on various forums. But
I couldn't find an exact duplicate of my bug description here. I hope
you find this interesting, as it took hours to investigate and write...
ProblemType: Bug
DistroRelease: Ubuntu 24.04
Package: xorg 1:7.7+23ubuntu3
ProcVersionSignature: Ubuntu 6.8.0-45.45-generic 6.8.12
Uname: Linux 6.8.0-45-generic x86_64
ApportVersion: 2.28.1-0ubuntu3.1
Architecture: amd64
BootLog: Error: [Errno 13] Permission denied: '/var/log/boot.log'
CasperMD5CheckResult: pass
CompositorRunning: None
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 11 09:08:39 2024
DistUpgraded: Fresh install
DistroCodename: noble
DistroVariant: ubuntu
ExtraDebuggingInterest: Yes, if not too technical
GraphicsCard:
Intel Corporation HD Graphics 530 [8086:1912] (rev 06) (prog-if 00 [VGA
controller])
Subsystem: Hewlett-Packard Company HD Graphics 530 [103c:8054]
Inst
