Bug 503376

Summary:	With Krohnkite active, changing monitor inputs messes with internal displays
Product:	[Plasma] kwin	Reporter:	Antti Savolainen <antti.savo>
Component:	multi-screen	Assignee:	KWin default assignee <kwin-bugs-null>
Status:	REPORTED ---
Severity:	normal	CC:	nate
Priority:	NOR	Keywords:	multiscreen
Version First Reported In:	6.3.4
Target Milestone:	---
Platform:	Arch Linux
OS:	Linux
Latest Commit:		Version Fixed In:
Sentry Crash Report:
Attachments:	Display configurations Display setup

Description Antti Savolainen 2025-04-26 10:22:09 UTC

Created attachment 180676 [details]
Display configurations

SUMMARY
Demonstrative video: https://youtu.be/-bNDuuOsuYc
I have a setup where some monitors are shared between my PC and laptop. One of the monitors mirrors the TV as is seen in the attached screenshot. In the video I have the rightmost monitor tuned to the laptop input and the game playing on the PC on the TV. When I change the input in the monitor's internal settings from laptop to PC, the game visuals get cut off like that. I then try to use Alt+Enter to resize the window and it freezes. 
Similar issues can be had when the laptop goes to sleep.
The issue arises from me working on something on my laptop while my partner is playing games on the TV. If I want to tune in to the her playing, I have to wait that she's in a safe area and I can not allow the laptop to sleep at any moment.

I'm assuming it's caused by the monitor turning off all of its inputs momentarily. If so, it could probably be fixed by kwin waiting one second before changing its internal display configuration when a monitor connection is dropped. For connecting a monitor such delay is probably not needed.

STEPS TO REPRODUCE
1. Share monitors between two machines
2. Have one of those monitors be a bit flaky about keeping up its connections
3. Have a game playing on one machine that is very sensitive to resolution changes
4. Change monitor input on the flaky monitor.

OBSERVED RESULT
Internal display state is altered

EXPECTED RESULT
Kwin should guard itself against momentarily dropped connections

SOFTWARE/OS VERSIONS
Linux: Arch Linux
KDE Plasma Version: 6.3.4
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0

Comment 1 Antti Savolainen 2025-04-26 13:45:43 UTC

And here is a phone recording of it happening IRL: https://youtu.be/9MQsVJ62oTg

Comment 2 Nate Graham 2025-04-28 15:48:42 UTC

Hmm, it seems like you've managed to put together a setup that's maximally unstable. :/

There are probably bugs we can fix here, but it seems like such a setup is playing with fire TBH.

Comment 3 Antti Savolainen 2025-04-29 10:50:36 UTC

Created attachment 180768 [details]
Display setup

Alright, I think I distilled the issue to its purest form. Scaling combined with input switching. Here I shut down the laptop and set the TV to be in a disabled state. I have only my two monitors enabled and the primary display has scaling. Now if I switch input source to something else and then switch back to the desktop input source from the monitor, I can reproduce the bug.

Comment 4 Antti Savolainen 2025-05-18 16:23:05 UTC

This could be an issue with Krohnkite. If I turn Krohnkite off, Elden Ring doesn't resize and it doesn't lock up. I'll leave it up to you if you want to fix this on Kwin side or if I should report this to Krohnkite.