Bug 515362

SUMMARY
libkscreen: Setting a custom mode and applying it via System Settings shows black screen with lagging mouse
Feature added in commit https://invent.kde.org/plasma/libkscreen/-/commit/4bbd79fcdce96dbf87ae5424397d02de38a5f841

STEPS TO REPRODUCE
1. Add a custom mode via kscreen-doctor (e.g. output.DP-1.addCustomMode.3440.1440.144000.full)
2. Apply the new resolution via System Settings

OBSERVED RESULT
Screen switches resolution but KDE enters some kind of failed state where only the mouse is visible and it is lagging very hard when moved

EXPECTED RESULT
Resolution 3440x1440@144 10-bit RGB is applied

SOFTWARE/OS VERSIONS
Linux: 6.18.8-3-cachyos (64-bit)
KDE Plasma Version: 6.5.91
KDE Frameworks Version: 6.22.0
Qt Version: 6.11.0

ADDITIONAL INFORMATION
All packages are up-to-date.
Display is an Alienware AW3423DWF connected via DisplayPort 1.4 to an RTX 3090.
This resolution is not officially exposed in the EDID (3440x1440@100 10-bit RGB is officially exposed)
The display supports a max resolution of 3440x1440@165 8-bit+FRC RGB or 10-bit YCbCr 4:2:2 (which is the default KDE chooses unless you run screen-doctor output.DP-1.wcg.disable which switches you to the RGB mode).
3440x1440@144 10-bit RGB works just fine in Windows. 
In Windows you can even do 3440x1440@165 10-bit RGB with non-standard timings: https://www.reddit.com/r/ultrawidemasterrace/comments/10w07qb/aw3423dwf_i_successfully_managed_10bit_at_165hz/