QML apps fall back to software rendering and produce visual bugs

[Ben M]

I am not a developer but a general Linux user. On Arch Linux with Device-2: NVIDIA GP106 [GeForce GTX 1060 6GB] driver: nvidia v: 525.89.02 video card, several QML GUI apps have serious visual bugs.

Notably Nextcloud: https://github.com/nextcloud/desktop/issues/5443
and KDE Kasts: https://bugs.kde.org/show_bug.cgi?id=466274

The Kasts developer was able to reproduce my problems by setting: QT_OPENGL=software QT_QUICK_BACKEND=software QML_SCENE_DEVICE=softwarecontext before running Kasts.

This makes me wonder:

1. Why is hardware rendering not being used in QML apps? I have confirmed hardware acceleration works in general following the mpv instructions here: https://wiki.archlinux.org/title/Hardware_video_acceleration#Verification
2. If hardware rendering is not working, why does software rendering produce these bugs?

More video settings:

 Display: x11 server: X.Org v: 21.1.7 with: Xwayland v: 22.1.8
    compositor: kwin_x11 driver: X: loaded: modesetting,nvidia unloaded: vesa
    alternate: fbdev,intel,nouveau,nv dri: iris gpu: nvidia,nvidia-nvswitch
    display-ID: :0 screens: 1