Created attachment 144015 [details]
Resolution mismatch under X11 and 100% global scale

SUMMARY
I have two monitors. The first with resolution of 3840x2160 and the second with 1680x1050. Before trying out Wayland the resolution listed for both monitors on the layout preview pane was correct and also relative dimensions were fine. Then I decided to try Wayland for the first time and I used the same resolutions but tried to set the scaling for the first only monitor to 1.5 while keeping 1.0 for the second monitor. Well since Wayland was not working as good as I expected (blurry text for example) I went back to using X11. But the layout preview pane now as some problems. The first monitor reports resolution of 2560x1440 in the preview pane, while it is still actually set to the original 3840x2160 via the dropdown setting below (the global scale is shown to be set to 100% correctly). Also the relative sizes in the preview pane are wrong. This causes a problem that if I try to position the smaller monitor to the left of the larger one and align the bottom edges then in reality the bottom edge of the left monitor is not aligned with the bottom edge of the right monitor, but it is actually a bit higher and you can not just move the mouse right along the bottom edges from right monitor to the left one. You have to go 720 pixels higher to be able to end up on the bottom of the left screen.


STEPS TO REPRODUCE
1. Have two monitors with different resolutions
2. In X11 position the one with lower resolution to the left of the one with higher resolution and align bottom edges
3. Make sure the X11 global scaling is 100% and the resolutions are reported correctly in layout preview pane
4. Logout and login with using Wayland
5. Set scaling of the monitor with higher resolution to 150% and use the same alignment as before, keep 100% scaling for the monitor with lower resolution
6. Switch back to using X11 and 100%

OBSERVED RESULT
The resolution of the monitor that had 150% scaling under Wayland is now shown incorrectly in layout preview pane and this pane does not present the right relative sizes/layouts of monitors.

EXPECTED RESULT
All monitors should have the rights resolution reported on X11 layout preview pane and relative sizes should be correct.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20211125
KDE Plasma Version: 5.23.3
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2
Kernel Version: 5.15.3-1-default (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Core™ i5-6600 CPU @ 3.30GHz
Memory: 31.3 GiB of RAM
Graphics Processor: AMD Radeon ™ RX 480 Graphics