Bug 480281

SUMMARY
Not sure, if this is even a kwin bug, or something else. But on my tablet device, when logged into the regular Plasma (Wayland) session, the appmenu of most GTK apps is broken in an awkward way, in that no matter where the appmenu is clicked, the click is always registered on the leftmost item.

This is not a huge issue when using a mouse, as hovering over a different item will select it, but when using the touchscreen, this essentially renders GTK appmenus completely unusable (there is a way to focus the correct item with some wizard-like finger movements, but it's incredibly unintuitive and cumbersome).

Note that this issue does NOT appear when window decorations are disabled via:
[org.kde.kdecoration2]
NoPlugin=true

As is the default for the mobile session (thus, in the mobile session, these appmenus work just fine).

When messing around with the appmenu, the affected app (I used gscan2pdf) spams this in the user journal:
Jan 24 17:42:43 nxtab ..gscan2pdf-wra[4552]: Tried to map a popup with a non-top most parent

STEPS TO REPRODUCE
Sorry, no idea. I'd say get a Microsoft Surface Go 3, install Plasma on it, and give it a try, but this doesn't seem viable.

Disabling the touch screen, changing screen orientation, and display scaling doesn't affect this issue however. The only thing that "fixes" it, is disabling window decorations entirely and forcing GTK apps to use CSDs.

OBSERVED RESULT
Appmenu click is always registered in the top left corner first (presumably)

EXPECTED RESULT
Appmenu click is registered at the position of the mouse cursor / touch

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: NixOS
(available in About System)
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.113.0
Qt Version: 5.15.12

ADDITIONAL INFORMATION
Since this may be difficult to reproduce due to potentially being hardware specific, I'm happy to test any patches or config changes thrown my way.