Created attachment 155655 [details]
Affected 1.25 + 1.5 screen layout

SUMMARY
With kwin_wayland 5.26.90 (bug wasn't present on kwin_wayland 5.26.5) maximized windows take also one pixel on other screen. I use fractional scaling, top-bottom screen arrangements (left-right wasn't tested). Bug reproduced with top×1.25 + bottom×1.5 and top×1 + bottom×1.5 screen layouts. One extra pixel is not kscreen problem, screens do not overlap (it is hidden by right panel on bottom screen and active maximized window on bottom screen).

STEPS TO REPRODUCE
1. Setup layout with two screens, bottom one has scaling 1.5, top could be any (reproduced at least with 1 and 1.25).
2. Maximize window on top screen (for easier noticing, contrast window and wallpaper may be used).
3. (Optional) Maximize some window on bottom screen.

OBSERVED RESULT
One pixel line overhang on on bottom screen from top max window.
With step 3 you could switch between windows and see how that extra line appears and hides.

EXPECTED RESULT
Maximized windows should not extend beyond their screen.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: openSUSE Tumbleweed 20230124 with KDE Plasma 5.27 beta
KDE Plasma Version: 5.26.90
KDE Frameworks Version: 5.102.0
Qt Version: 5.15.8+kde160

ADDITIONAL INFORMATION

This bug looks like some sort of opposite to https://bugs.kde.org/show_bug.cgi?id=459373
Unlike that bug it doesn't depend on XWayland (native Wayland clients are used) and it wasn't present before 5.27 beta.

example of affected layout (full in attachment)
`kscreen-doctor -j | jq '[.outputs[] | { name: .name, pos: .pos, size: .size, scale: .scale }]'`
```
[
  {
    "name": "eDP-1",
    "pos": {
      "x": 385,
      "y": 1152
    },
    "size": {
      "height": 1080,
      "width": 1920
    },
    "scale": 1.5
  },
  {
    "name": "DP-1",
    "pos": {
      "x": 0,
      "y": 0
    },
    "size": {
      "height": 1440,
      "width": 2560
    },
    "scale": 1.25
  }
]
```