Created attachment 189164 [details]
Screen arrangement.

SUMMARY
When moving a (non-maximized) window to the screen edge, the window border (and shadow) are visible on the other screen as a thin line with a shadow right on the edge of a screen.

STEPS TO REPRODUCE
1. Have two screens connected.
2. Move a window to the screen edge where the two screens meet. Using edge snap zone (under Window Management -> Window Behavior -> Movement) helps here a lot.
3. Observe the thin line and a shadow appear on the second screen.

EXPECTED RESULT
The 1 pixel (if my primary display is at 150% scaling) border shouldn't be visible on the second screen.

SOFTWARE/OS VERSIONS
Operating System: KDE Linux 2026-02-02
KDE Plasma Version: 6.6.80
KDE Frameworks Version: 6.23.0
Qt Version: 6.10.2
Kernel Version: 6.18.7-zen1-1-zen (64-bit)
Graphics Platform: Wayland
Processors: 4 × Intel® Pentium® CPU 4405U @ 2.10GHz
Memory: 8 GiB of RAM (7.2 GiB usable)
Graphics Processor: Intel® HD Graphics 510

- AND ALSO ON MY MAIN SYSTEM
Operating System: EndeavourOS 
KDE Plasma Version: 6.5.5
KDE Frameworks Version: 6.22.0
Qt Version: 6.10.1
Kernel Version: 6.12.67-1-lts (64-bit)
Graphics Platform: Wayland
Processors: 32 × AMD Ryzen 9 7950X3D 16-Core Processor
Memory: 32 GiB of RAM (31.0 GiB usable)
Graphics Processor: AMD Radeon RX 7900 XTX


ADDITIONAL NOTES:
In attachments you can see the bug and my display arrangement (the bug appear both with 100% and 150% scaling)

I'm unsure if this would be a "visual bug" or a feature/wishlist request. In any case, I think the border and shadow should be hidden from other screens until the window actually crosses onto other screens.