Bug 511592

Summary: Maximizing a window on a monitor briefly flashes the window on secondary monitor to the right
Product: [Plasma] kwin Reporter: moichumoigaming
Component: coreAssignee: KWin default assignee <kwin-bugs-null>
Status: REPORTED ---    
Severity: normal    
Priority: NOR    
Version First Reported In: 6.5.0   
Target Milestone: ---   
Platform: CachyOS   
OS: Linux   
Latest Commit: Version Fixed/Implemented In:
Sentry Crash Report:

Description moichumoigaming 2025-11-03 23:19:28 UTC 
SUMMARY
When maximizing a window on a left side monitor, having another monitor to the right, the window will occasionally briefly flash on the second monitor.

This is especially apparent when the monitor on which you are maximizing the window is scaled up, and the right side monitor isn't. The maximized window is very big, in pixel size, on the scaled monitor, and therefore takes up a good bit of the second monitor's screen during the flash.

I have 3 monitors, left to right scaling : 150 %, 150 %, 100 %, and the issue happens as well when maximizing a window on my left most monitor. The flashing appears on my middle monitor, which is also scaled at 150 %.

The bug doesn't happen 100 % of the time, and I don't know the exact trigger to make it happen 100 % of the time. It seems random.

STEPS TO REPRODUCE
1. Have two monitors.
2. Open firefox in a window that is not maximized, on the left most monitor.
3. Maximize the window.

Important note : I have all relevant desktop effects disabled, and my global animation speed is instant.

OBSERVED RESULT
You should see, at some point, a flashing of the window on the right monitor. If not, try moving the window around, and try again. It should happen at some point.
There is also a frame for which the window is at its windowed size, in the top left corner of the left monitor, before being maximized.

EXPECTED RESULT
The window should maximize without flashing on secondary monitors.

SOFTWARE/OS VERSIONS
Operating System: CachyOS Linux 
KDE Plasma Version: 6.5.1
KDE Frameworks Version: 6.19.0
Qt Version: 6.10.0

ADDITIONAL INFORMATION
Comment 1 moichumoigaming 2025-11-03 23:26:59 UTC 
The bug does not happen with all applications it seems, but i've had it happen with at least two different applications :
1. Firefox.
2. Looking glass client (for virtual machines).

These two applications were fine before upgrading to 6.5.