SUMMARY
I am using KWins BorderlessMaximizedWindows with a top panel on each screen. (with plasmoid applet-window-buttons6)
When unmaximizing windows, there is a visual glitch on the second monitor (or vice versa).
The glitch gets worse when enabling the effect: Maximise (Kinetic).
If I disable BorderlessMaximizedWindows, the glitch is gone.
It is affecting me on both X11 and Wayland.

STEPS TO REPRODUCE
1. Add a top Panel to the Desktop Primary Screen.
2. Attach a second Monitor.
2. Add a way to maximize and unmaximize Windows, when KWin BorderlessMaximizedWindows is set to true. (e.g. use plasma6-window-title-applet from dhruv8sh and double click on it or toogle maximize unmaximize with keyboard shortcuts.
3. Edit .config/kwinrc and enable BorderlessMaximizedWindows
[Windows]
BorderlessMaximizedWindows=true
4. Reload KWin and maximize and unmaximize windows. 

OBSERVED RESULT
The animation leaks to the second screen and creates a flicker glitch on it.

EXPECTED RESULT
The animation should not leak to the second screen, like when not using borderless maximized windows.



SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 6.12.41-1-longterm on openSuse Tumbleweed
KDE Plasma Version: 6.4.3
KDE Frameworks Version:  6.16.0
Qt Version: 6.9.1