I can reproduce this bug, but with an interesting detail: Only on one of my two identical screens.

Similar to David, I can set my Applet to 100%, but it only corresponds to 20% of the monitor brightness as shown on the monitor OSD. Like David if I then change the applet to 50%, it is actually 10% brightness

If I then change the monitors OSD brightness to 100%, the applet "controls" correctly, but it does not update the montior OSD or knowledge of the changes, showing still at 100% brightness and full power use, regardless of the applet setting,

On my other monitor it works as intended, applet settings directly update and adjust the Monitor OSD brightness settings, and correspond correctly.

Operating System: Fedora Linux 41
KDE Plasma Version: 6.3.0
KDE Frameworks Version: 6.10.0
Qt Version: 6.8.2
Kernel Version: 6.12.11-200.fc41.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 16 × Intel® Core™ i9-9900K CPU @ 3.60GHz
Memory: 31.1 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 1080 Ti

2x Dell U2515H monitors, connected via Display port

I am happy to help investigate further if I am given information for how to diagnose or next steps to try.

I should mention tbat my monitors are all Dell S2721QS models, in case it's relevant. Also, I should emphasise that this is a regression from Plasma 6.2.

> If I then change the monitors OSD brightness to 100%, the applet "controls" correctly, but it does not update the montior OSD or knowledge of the changes, showing still at 100% brightness and full power use, regardless of the applet setting,

This is a bit different for me - if I set the monitor to 100% brightness via the OSD, it goes straight back to a value between 0% and 30% when I use the applet controls again.

I've disabled the feature with POWERDEVIL_NO_DDCUTIL=1 for now.

I'm having the same issue with a HP OMEN 27q monitor as my primary monitor. Whenever the system wakes from sleep, the primary monitor will set its brightness to 30%, but the "Brightness and Colour" widget will show it as 100%. This does not seem to effect my secondary monitor, which is a smaller monitor from the same brand. This did not occur in 6.2.

---

Operating System: Fedora Linux 41
KDE Plasma Version: 6.3.0
KDE Frameworks Version: 6.11.0
Qt Version: 6.8.2
Kernel Version: 6.12.11-200.fc41.x86_64 (64-bit)
Graphics Platform: Wayland
Graphics Processor 1: AMD Radeon RX 6700 XT

Hi - I can't reproduce this issue myself, but leaving a comment for possible reference by others - I wonder if this is potentially related to the following bug: https://bugs.kde.org/show_bug.cgi?id=495003 (since the bug mentioned by Kath seems to be slightly different)?

I'm able to replicate this on a laptop with Plasma 6.3.2 and an nvidia GPU
2 monitors are attached via a dock (One over DisplayPort, the other over HDMI)
I set the brightness of all displays to 100% via the Brightness applet

Actual brightness shown in monitor settings
LG 34GK950F - DisplayPort - 90 (It looks noticeably less bright than the other two displays)
LG Ultragear - HDMI - 100

Please attach the output of kscreen-doctor -o while the issue is present.

Monitor brightness right now:
| Screen |  Widget %  | Monitor settings  |
| Laptop  |  70  | 70 |
| Mon 1  |  50  |  90 |
| Mon 1  |  50  |  100 |

❯ kscreen-doctor -o | grep -E "Output|Geometry|Scale|Bright"
Output: 1 eDP-1
        Geometry: 0,0 2560x1600
        Scale: 1.5
        Brightness control: supported, set to 70% and dimming to 100%
Output: 2 DP-9
        Geometry: 6000,80 3440x1440
        Scale: 1
        Brightness control: supported, set to 50% and dimming to 100%
Output: 3 DP-10
        Geometry: 2560,80 3440x1440
        Scale: 1
        Brightness control: supported, set to 50% and dimming to 100%

Full output attached

Created attachment 179150 [details]
kscreen-doctor output

hmm, that looks more like the brightness isn't getting applied at all, rather than the maximum being wrong.

*** Bug 501510 has been marked as a duplicate of this bug. ***

Unfortunately a fresh install of KDE Plasma 6.3.4 (Fedora 42) hasn't improved this behaviour on my machine - my monitors are all still capped at 30% brightness when using the software controls (incidentally, the controls weren't actually visible until I restarted plasma-powerdevil.service).

(In reply to David Warner from comment #11)

> *** Bug 501510 has been marked as a duplicate of this bug. ***

This has been remediated for me with my AOC Q3279WG5B, since I posted that bug:

> ~~~
> Operating System: Fedora Linux 42
> KDE Plasma Version: 6.3.5
> KDE Frameworks Version: 6.14.0
> Qt Version: 6.9.0
> Kernel Version: 6.14.8-300.fc42.x86_64 (64-bit)
> Graphics Platform: Wayland
> ~~~

I didn't even need to reinstall.

This also seems to be fixed for me on my Omen 27Q and Fedora 42. I have noticed sometimes they brightness sliders don't show up at all, but can't remember if that was before or after I applied the 6.3.5 update.

That being said, I can't see a commit listed for 6.3.5 that would have actually addressed the issue, and the kwin/powerdevil changes for 6.3.4->5 don't seem be relevant, so not sure if it's resolved or just hiding!

> ===
> Operating System: Fedora Linux 42
> KDE Plasma Version: 6.3.5
> KDE Frameworks Version: 6.14.0
> Qt Version: 6.9.0
> Kernel Version: 6.14.8-300.fc42.x86_64 (64-bit)
> Graphics Platform: Wayland
> ===

Hi David - just checking, as the original reporter of this issue, are you seeing it still occur with Plasma 6.3.5?

(In reply to John Kizer from comment #14)
> Hi David - just checking, as the original reporter of this issue, are you
> seeing it still occur with Plasma 6.3.5?

Sorry for the delay - happy to confirm it's working as intended. 100% brightness in the applet maps to 100% brightness in the OSD controls again.

(In reply to David Warner from comment #15)
> (In reply to John Kizer from comment #14)
> > Hi David - just checking, as the original reporter of this issue, are you
> > seeing it still occur with Plasma 6.3.5?
> 
> Sorry for the delay - happy to confirm it's working as intended. 100%
> brightness in the applet maps to 100% brightness in the OSD controls again.

Unfortunately I might have spoken too soon - everything was working perfectly after restarting, but after letting the monitors go to sleep, Powerdevil got into a bad state and the applet stopped displaying brightness settings entirely. In the logs I see things like

> Jun 16 11:43:46 fedora org_kde_powerdevil[2907]: org.kde.powerdevil: [DDCutilDisplay]: ddca_open_display2 -3030
> Jun 16 11:43:46 fedora org_kde_powerdevil[2907]: org.kde.powerdevil: [DDCutilDisplay]: "DELL S2721QS" failed to set DDC/CI brightness
> Jun 16 11:48:29 fedora org_kde_powerdevil[2907]: org.kde.powerdevil: Adjust screen brightness step failed: no displays available to adjust

This might be a separate issue altogether.

(In reply to David Warner from comment #16)
> Unfortunately I might have spoken too soon - everything was working
> perfectly after restarting, but after letting the monitors go to sleep,
> Powerdevil got into a bad state and the applet stopped displaying brightness
> settings entirely. In the logs I see things like
> 
> > Jun 16 11:43:46 fedora org_kde_powerdevil[2907]: org.kde.powerdevil: [DDCutilDisplay]: ddca_open_display2 -3030
> > Jun 16 11:43:46 fedora org_kde_powerdevil[2907]: org.kde.powerdevil: [DDCutilDisplay]: "DELL S2721QS" failed to set DDC/CI brightness
> > Jun 16 11:48:29 fedora org_kde_powerdevil[2907]: org.kde.powerdevil: Adjust screen brightness step failed: no displays available to adjust
> 
> This might be a separate issue altogether.

Heya, I reported that issue here: https://bugs.kde.org/show_bug.cgi?id=505190