Bug 509559

Summary:	Applying settings for external monitor (mirrored) does not work
Product:	[Plasma] KScreen	Reporter:	Bernhard Breinbauer <bernhard>
Component:	common	Assignee:	kscreen-bugs-null <kscreen-bugs-null>
Status:	RESOLVED DUPLICATE
Severity:	normal	CC:	nate, xaver.hugl
Priority:	NOR	Keywords:	wayland-only
Version First Reported In:	6.4.5
Target Milestone:	---
Platform:	openSUSE
OS:	Linux
Latest Commit:		Version Fixed In:
Sentry Crash Report:
Attachments:	Display settings

Description Bernhard Breinbauer 2025-09-16 16:45:04 UTC

SUMMARY
When connecting an external monitor (actually projector over HDMI) and set it as mirror of the internal monitor, I cannot apply any settings to the external monitor. The dialog asking to keep the settings does not appear.
The external monitor is set to a very low resolution and cannot be changed. Setting the monitor mode to extension left/right allows changing the resolution.

Some observations:
* Happens only under wayland, works in X11
* Did work in releases <= 6.3
* Interestingly, applying settings works when the monitor is rotated by 90°
* Checked with different kernel versions and HDMI adapters
* Checked with several different projectors, same behavior. I found one external monitor where it works as expected.
* Checked with Neon Live System, same problem

STEPS TO REPRODUCE
1. attach external monitor
2. set to mirror
3. try to set the resolution in systemsettings -> does not work

OBSERVED RESULT
Applying a different resolution does not work. The dialog asking to keep the setting does not appear.

EXPECTED RESULT
Sucessfully changing the resolution of a mirrored monitor

SOFTWARE/OS VERSIONS
KDE Plasma Version: 6.4.5
KDE Frameworks Version: 6.17
Qt Version: 6.9.2

Comment 1 Bernhard Breinbauer 2025-09-17 16:39:59 UTC

I did some more testing today on a Panasonic Projector, to which I can connect over HDMI and VGA.
* Connection over HDMI shows the problems described above
* Connection over VGA works (immediately, without further configuration required)
Please note, that both are using the same USB-C to HDMI adapter. The working variant is using an additional HDMI-to-VGA adapter.
In both cases, the projector is detected correctly (see attachment).

The resolutions listed in the settings dialog are the same in both cases, but the HDMI variant defaults to 720x480 while the VGA variant defaults to 1680x1050.

Comment 2 Bernhard Breinbauer 2025-09-17 16:41:29 UTC

Created attachment 185015 [details]
Display settings

Comment 3 Zamundaaa 2025-09-17 17:34:31 UTC

This is a consequence of how mirroring is implemented - changing the output mode, size and position to attempt to match the geometry of the source display. You were only able to change the mode in the past because this was implemented as an even larger hack in KScreen.

Once we don't modify the mode anymore, this issue will go away.

*** This bug has been marked as a duplicate of bug 481222 ***

Comment 4 Bernhard Breinbauer 2025-09-18 09:20:20 UTC

Thanks for the reply. 
I tested with today's KDE Neon unstable build and the problem persists. So, I'm a bit anxious the situation will be the same in Plasma 6.5.
Is there some workaround/patch in the meantime, which I could try? I'm currently quite handicapped in my daily work.